You gain profit from your ticket booth. The bigger your zoo gets -the bigger the profit. Ticket booth money can be collected every 15 minutes and will continue to fill even when you're not active in the game. The amount paid depends on Zoo Happiness, and the number of daily visitors.
Get more daily visitors by getting more animals and items. Get more money once a day by shaking trees, upgrading some of your kiosks for larger profits, and visiting your friends' zoos. Posting animals to your feed and helping them find homes will also give you a bonus.