Lockers are located at the entrance to each park. A locker costs $5/day. You also pay a $2 deposit on each key that you get.
So, let's say you start the day at the Magic Kingdom. The lockers are underneath the train station. You'll pay $7.
At about noon, you decide to leave for a break. You'll get your stuff out of your locker, take the key back, and they'll give you $2, and a receipt. After your break, you decide to go to Animal Kingdom. The lockers are on the left as you enter. Show them your receipt, give them $2, they'll give you a key.