There are two types of currency in Tiny Death Star. Credits and Imperial Bux.
Sometimes called Coins, Credits are the main currency of the game. Credits are earned when Bitizens make purchases on commercial levels, and when they pay rent. Players also earn credits using the elevator to deliver Bitizens to a level and completing Imperial Missions. Players spend credits to build new levels and when starting assembly of Imperial items.
Imperial Bux are the secondary currency. Bux can be earned as a reward for finding spies, as a tip for finding a Bitizen, for giving a Bitizen their dream job, for building a level, or for opening the game on a Bitizen's birthday. They can also be purchased using real-world funds. Bux can be spent on speeding up improvements, unlocking costumes, Bitizens, levels, elevators, and more.