After you graduate, leave school, or drop below half-time enrollment, you will have a six-month "grace period" before you begin repayment. During this period, you'll receive repayment information, and you'll be notified of your first payment due date. You're responsible for beginning repayment on time, even if you don't receive this information. Payments are usually due monthly.
During the grace period on a subsidized loan, you don’t have to pay any principal, and you won’t be charged interest. During the grace period on an unsubsidized loan, you don’t have to pay any principal, but you will be charged interest. You can either pay the interest or it will be capitalized (added to your principal loan balance, thus increasing the amount you’ll repay).