Every faction has it's own specific currecny, but the one used and accepted everywhere is the Trader's Credit. Trader's Credits, or TCs, are a good thing to have around when you're visiting a planet controlled by another faction. However, it's a good idea to make sure you have plenty of them, since there's customs to go through and everything. An added bonus is that every level you'll recieve a bonus of TCs. 200 per level, and every tenth level, you'll get 1000 TCs. Any questions, you know where to put them.