I think most schools send you a form to send back with a deposit, and you check that you want them to hold your acceptance. This is required so they know you are still interested in the school. If you don't send it back, they'll give your spot to somebody else. If you do, they know you might come there. Some schools refund the deposit if you decide not to go, but others don't. Then on April 15 (I think?) you have to decide which one of your acceptances you want to hold on to, and withdraw from the rest. You can keep waitlists, though.