Money doesn't just make the world go 'round — it keeps the Galaxy spinning, too.
The official currency of the Terran Empire is the credit. A handful of Senatorial worlds have their own currency, even there credits constitute legal tender, but only credits are valid for Imperial taxes and interstellar trade. Credits have no physical form, but citizens can store them on credisks — thin disks of nearly indestructible plastic with a liquid crystal holographic display showing the value stored on the disk. The holder of a credisk can plug it into any standard bank or commercial exchange terminal to add or subtract value from it; that's how he buys goods and services, receives his pay, and so forth. People also exchange credisks directly.
A credisk's built-in data crystals keep a record of every transaction conducted, which means Imperial authorities can trace the disk's use. Criminals, rebels, and others who wish to keep their dealings hidden often avoid credisks for this reason.
Other governments have different monetary systems. The Perseid Empire uses synthetic jewels called vorads ("stars"). The Conjoined Civilizations Republic has the Republic Trade Unit (RTU), similar in most respects to the Imperial credit. The Velarian Confederation uses Velarian lhendi ("rings"); the Ackalians use halku ("marks"). All these currencies exist in both physical and electronic form — except for the RTU, an entirely electronic type of money. Surprisingly, neither the Mon'dabi nor the Thorgons use money as such.
The Mon'dabi never evolved the idea of a government-backed currency; instead, all transactions involve the barter of goods and services or promissory notes signed by individuals and banks. Among themselves, Mon'dabi use individual notes for smallscale transactions and bank or corporate notes for interstellar commerce. With aliens or someone whose personal note they cannot trust, they only barter. This means almost all business transactions require extensive haggling as the two parties find some combination of promissory notes and objects each considers an acceptable exchange. Most Mon'dabi households have an odd stockpile of items acquired in barter, awaiting use in future trades.
The Thorgons have such a planned and centralized society they don't need money. People do their jobs because they must, and they get the supplies they need, with bonuses for good performance. Only the government can conduct trade with outside entities, and it only barters (but does stockpile foreign money for use in bribes and purchases). A thriving underground economy using a mix of Perseid, Terran, and Ackalian money exists on most Thorgon worlds.
The Varanyi use their own currency, the she'mra, only within their empire. When they buy from or sell to others, they always use the other person's native currency. Where they come by their stocks of currency remains unknown.
In most campaigns, the easiest thing for the GM to do is assume all Galactic currencies are roughly equal in value. Gaming groups desiring more "real ism" and complexity can use the accompanying Exchange Rate Table to determine the relative value of currencies. It indicates the average exchange values during the reign of Marissa 111, though the GM may adjust the rates slightly from game to game to represent standard fluctuations in the Galactic economy. Gamemasters who set their campaigns during other times may want to alter the rates. In troubled areas or worlds far from the centers of Galactic life, exchange rates may vary... but almost never in the PCs' favor.