As in almost every game of its kind, in Zoo Evolution you have to complete certain missions in order to earn the corresponding rewards. These missions normally involve building new spaces for the animals, leveling your animals up, and other similar tasks.
In order to carry out all your plans, you need three different types of currency: zoo coins, tickets, and candy. Both the coins and the tickets will let you buy new spaces for your animals in the zoo, while the candy will help your animals level up.
Zoo Evolution is a casual game that, while not especially original, offers a very entertaining gameplay. The game also has some really nice graphics.