You gotta have cash even in the future, otherwise you can't buy anything. Utopian socialist paradises excluded of course.
Traditionally, the unit of currency in science fictional futures is called the "credit". This is the futuristic equivalent of a dollar, Euro, or whatever. Using metric, one megacredit is a cool million credits.
And there are a few examples of science fictional socialist utopias where money is obsolete, e.g., Star Trek. Good luck with that.
You can find an amusing list of the names of various fictional currencies here
- In SPI's RPG Universe and Star Trader, the unit of currency was the "Tran" or "transaction", where 1 Tran was equal to about $500.
- SPI's Star Force had "LaborCredits".
- In Karl Gallagher's Torchship the unit of currency is called Keynes or "Keys", named after Keynesian Economics.
- In Philip E. High's The Prodigal Sun money was literally hours of work.
- EVE Online has a little more complex a take on things. The currency, known as ISK (Inter-Stellar Kredits), is not so much a global currency as it is a global exchange currency. Planetary economies and sometimes individual planetary nations almost all have their own currencies, ISK was merely setup as an exchange medium to manage the obscene amounts of money being used at the interstellar level
- In the Micronauts series of comic books, the evil Baron Karza has a monopoly on lifespan prolongation technology (the "Body Banks"). He issues his own currency called "Life Credits", with which a person can purchase extended lifespan. The underclass waste all their credits in gambling institutions, and can sell personal organs and other body parts in exchange for more life credits. The aristocracy is firmly under control of Karza, since they know he literally has the power of life or death over them.
- In The Great Explosion by Eric Frank Russell, the planet K22g is a post-money utopian society, but they still have a medium of exchange. They use favor-exchange based on "obs" (obligations). This might explain the value of the poker chips you see in all those Star Trek poker games.
- In John Morressy's Del Whitby series, the unit of currency was the cash-cube. These were cubical coins of precious metal which would stack into neat rectilinear piles.
- In Diane Duane's My Enemy, My Ally, the Romulan's currency is in the form of chains of precious metal.
- In John Brunner's Intersellar Empire series, the currency is in the form of rings of preciouis metal.
- In Frank Herbert's novel DUNE, the Fremen's currency is based on liters of water, symbolized by metal rings. They tie the rings in strips of cloth so as to not make noise when they are sneaking up on an enemy.
- In the Battletech universe, a common unit of currency was the C-Bill, redeemable for a certain amount of data transmission on Comstar's FTL communications network.
- In the simulation game High Frontier, the unit of currency is "the most valuable thing in the universe", namely water. Water can be used for reaction mass, as a source of hydrogen and oxygen, radiation shielding, and a host of other uses. The unit is a 40 metric ton tank.
- In Kim Stanley Robinson's Red Mars Trilogy, calories of heat were used as the basis of the Martian economy.
- I am somewhat dubious about the Quasi Universal Intergalactic Denomination. Apparently it is intended to be safe in the space environment and will survive the space environment. This means it is constructed out of a space-qualified polymer, emit no toxic fumes, has no sharp edged, be resistant to high temperatures, and not use a magnetic strip like a credit card since cosmic radiation will render them inoperative.
- Of course nowadays most people use credit cards and PayPal.
Money has four functions:
- Medium Of Exchange: a way to avoid the double coincidence of wants problem inherent in the barter system. Or "he wants two coconuts for that shirt but all I got is this mango."
- Common Measure Of Value (or Unit Of Account): a way to set a standard price tag on market value of goods, services, and other transactions. This is essential to allow commercial agreements that involve debt. The loan officer ain't gonna give you a loan for that used car if you are using the barter system. It is too hard to figure the car's value in tiger-skins.
- Standard Of Value (or Standard Of Deferred Payment): using money to settle a debt sometime in the future. As Wimpey says "I'll gladly pay you Tuesday for a hamburger today"
- Store Of Value: So you can bank your money, with the expectation that when you withdraw the money it is still worth something. This doesn't work with the Banana Standard, put a banana in a bank box and you will be dismayed to find in a few years it has turned into worthless moldy goo.
Note that while Planet A and Planet B may both internally use a barter system instead of money, they might use arbitrary money (a nonstandard medium of exchange) when trading between each other. Using a medium of exchange avoids the problem of ensuring there is a double coincidence of wants, which is a problem inherent in the barter system.
The point is that a double coincidence of wants does not happen very often, which makes the barter system rather awkward and impractical. If however you create some arbitrary money-like medium of exchange, you can avoid the whole mess. It also avoids the problem of when one of the things you are bartering has a shelf-life or is seasonal.
Sometimes you will see traders using Trade Tokens. These are basically money that is not issued by a government, but instead by a private company, group, association or individual. From the 17th to the early 19th century these were used by merchants because the local government was not up to the task of issuing enough coins to allow business to operate. Nowadays you generally see them in the form of casino chips, in video arcades, and car washes. But a hypothetical interstellar trading company might issue their own trade tokens if there were no local government in the trade area, or at least one single government recognized by all the trade planets.
And scrip is an even more localized form of trade tokens. You often see this in old time mining or logging camps. The employees were not paid in money, but instead in company scrip. The company scrip could only be spent in the company store. Due to this Truck system, the employees more often than not wound up owing their soul to the company store.
Historically coins were disc shaped because that's what you get when you roll out a measured ball of precious metal and flatten it with a stamp bearing the King's face. They are also convenient to carry, especially if they have a hole in the middle for stringing. You should mill the edges to prevent crooks from engaging in the crime of coin clipping (this does little to stop the crime of "sweating" coins). Though nowadays most US coins are composed of such worthless metal that the milling is purely decorative or as a aid to the visually handicapped.
Gold coins are generally composed of a gold alloy (Coinage Gold) because pure gold is too soft to hold up to the stress of being used as a coin (and makes it too easy for criminals to clip or sweat the coins). In modern times, a popular popular choice for silver-colored coins is the alloy cupronickel, due to its corrosion resistance, electrical conductivity, durability, malleability, low allergy risk, ease of stamping (metalworking), antimicrobial properties and recyclability.
In low-tech places, coins are manufactured by hammering or casting. In more modern-day tech settings, coins are machine-struck. In futuristic data-driven societies, physical coins are obsolete and instead virtual coins are used. Indeed, as previously mentioned, physical money may actually be illegal since the Police State cannot trace physical money transactions.
In medieval times there were so many currencies that merchants had to carry coin pan balances in order to determine the worth of a given coin.
There are three basic monetary systems:
- Commodity money: The currency is physical lumps of something valuable, e.g., solid coinage gold coins.
- Representative or Commodity-backed money: The currency is paper bills or worthless coins that are backed by stores of something valuable in the treasury. Example: the Gold standard. Paper gold certificates can only be printed by the treasury up to the amount of gold in their vaults. Most nations on Terra abandoned the gold standard during the 20th century due to the inherent problems. Most current economists are not in favor of a return to the gold standard, and all efforts in the US to legislate such a return have failed.
- Fiat money: when the paper bills have value because the issuing government says so. Most modern governments use fiat money because it allows flexibility during a financial crisis. It allows the government to adopt an expansionary monetary policy aka "printing money".
Unless all the planets you trade with are members of the same interstellar government, or there exists some sort of interstellar money-changing organization, the money used on one planet is worthless on another planet.
If you have a group of planets that share a common currency, for the planets sake it is vitally important that they share a common fiscal policy. The ongoing Eurozone crisis has been made much worse by the fact that while the Eurozone has monetary union (i.e., one currency, the Euro) it does not have fiscal union (e.g., different tax and public pension rules). This ties the hands of European leaders, making the crisis almost impossible to solve. When the Eurozone was proposed, the various nations were persuaded to surrender their currency, but reluctant to surrender control of their fiscal policy (give up their national sovereignty? Never!). The proponents figured to get around the problem by doing the Eurozone union in two stages, which in retrospect was an insanely bad decision.
Debasing a currency means making a nation's money worth less. Generally this is done by the nation in question in order to gain some financial advantage at the expense of its citizens. The historic method was to secretly reduce the precious metal content of each coin, assuming a government unsophisticated enough to actually think that the gold standard is a good idea. If the goverment is not on the gold or other valuable element standard, it can engage in the more modern method of currency debasement by printing more paper money.
Soon Gresham's law rears its ugly head. The old more valuable coins will vanish from circulation as people horde them, and the new less valuable coins will become widely used.
Obviously criminals will engage in do-it-yourself unofficial currency debasement, aka "counterfeiting". If the local government is on the gold standard the crooks will clip the coins, basically reducing the gold content of the coin by making the coin smaller, and melting the little trimmed or filed off bits into ingots. If the govenment uses paper money it is time for the eternal war between crooks cranking out counterfeit money with commercially available printing technology (paper and 3D) and governments trying more and more high-tech anti-counterfeiting measures. Counterfeiting is always illegal, and the penalties are severe. The government is of the opinion that it is the only one authorized to debase the currency, and it is real touchy about unfair competition.
And some reckless criminals will engage in the insanely dangerous crime of counterfeiting those trade tokens known as "casino tokens". This is outrageously risky for two reasons:  the tokens have anti-counterfeiting measures and counterfeit detection technology orders of magnitude more sophisticated that government measures, and  since many casinos are owned and operated by organized crime they will simply murder you instead of bothering to call the cops.
In science fiction debasement sometimes happens for more radical reasons. Some science fictional means suddenly destroys the value of the material used for the standard; e.g., some idiot invents a device that cranks out tons of gold, thus blowing the gold standard out of the water and crashing the entire economy. Alternatively another idiot invents some new and improved method of counterfeiting money, commonly by inventing a matter-duplicator which can make perfect duplicates indistinguishable from the original.
- In James Blish's Cities in Flight the OC Dollar was based on the Germanium Standard, because of its vital use in transistors and computer chips. This worked fine, until some joker figured out how to synthesise germanium and thus obliterated the economy of the whole galaxy.
- In George O, Smith's "Pandora's Millions", the invention of a replicator crashed the economy of the solar system. Replicators mean there are no longer any rare metals to base your money on, and all material goods become basically free. The only thing of value are personal services (such as those of a surgeon or doctor). The only thing that prevents utter disaster is a synthetic element that cannot be replicated (because replication causes it to explode). The element allows one to create cheques, legal tender, and other critical items that cannot be counterfeited by a replicator.
- In the Star Trek universe, the Federation is a post-money society that uses replicators. The Ferengi use "gold-pressed Latinum" as the basis of their currency, since Latinum is the one element a replicator cannot create.
- In the Demon Princes pentology by Jack Vance the currency 'SVU' or Standard Value Unit was a printed note equal in value to one hour of common labor. A device called a "fake meter" is used to detect counterfeits. In the second novel the protagonist discover how to fool the fake meter, and hilarity ensues.
In some science fiction, physical money is illegal since the Police State cannot trace physical money transactions (see Cashless Society). By law all money transations must be via credit cards or debit cards. Intangible electronic numbers transferred from one bank ledger to another through traceable internet connections (credit cards or debit cards), so the state can keep a close eye on all your purchases. To the very last penny.
Naturally people wanting to bribe somebody or purchase illegal goods had to find some physical untraceable alternative. Purchasing illegal drugs with your bank card will just get you busted by the narks.
Such illegal currency is related to the concept of money laundering.
In the science fiction show The Expanse, Detective Miller accepts bribes in the form of casino chips. One of the authors said: "Why do they use casino chips as money on The Expanse? It's how you get a black market economy when everything else is traceable e-money."
In the real world, events overtook science fiction when some genius invented a way to transfer money over the internet, but encrypted in such a way that the authorities could not track it. Specifically they can see the transaction but have no idea of the identity of the sender or receiver. This is called Bitcoin (BTC), symbolized by , Ƀ, or ฿. Bitcoin is the first cryptocurrency that is decentralized.