There are two forms of currency, credits (₡) and platinum and where they can be spent. Generally speaking there is fairly stable exchange rate between credits and platinum, but that really depends on where you are at and what you are buying. For example, in the core worlds, your only option is credits and paper currency. In the core worlds, it is illegal to use the untraceable currency and could result in jail time if caught. In the core worlds, the strong suggestion is to hit the money changer right after hitting the pad.

Out on the rim, however, that's a different story. Most people don't want to have to deal with the Alliance and their credit accounts because of the overhead and the bureaucracy, so coins are all you need.

Basic rate of exchange:
2.5 Platinum = ₡1 and 1 Platinum = ₡0.4
5 Gold = ₡1 and ₡0.2 = 1 Gold