Credits

Credits are the universal currency of the galaxy, shared by all races. Credits are used to pay upkeep costs each turn, rush production, cover the costs of diplomatic deals, and cost overruns due to having fleets larger than the command points pool.

Earning credits

 * The principal source of money is taxation. Every population unit generates a set amount of cash each turn, according to the tax rate. Some structures also provide an additional amount of income regardless of population size.
 * Diplomatic deals also allow for money to exchange hands, as lump payments or tribute paid per turn. In general, expecting the other party to buy your goods can be hit and miss, but once rich enough, you can demand tribute from races that are afraid of your might and get away with it. Some races will even offer payments to keep you at bay (refusing one is tantamount to a declaration of war, however).
 * Finally, trade goods production is the last major source of cash. The caveat is that it precludes producing other, useful items on a given colony.