If you have a Direct Subsidized Loan, there will be no interest charges while you are enrolled in school at least half-time or during deferments. Interest for this loan begins once you start your repayments. If you have a Direct Unsubsidized Loan, interest begins accruing the day the loan is disbursed to you and continues until the day you fully repay the loan. You may pay the accumulating interest while you are in school, or you can choose to capitalize the interest.
NOTE: There is also a loan origination fee, which is another expense to borrowing a Direct Loan. This fee is about 1% and is charged for both subsidized and unsubsidized loans. This fee is subtracted from the loan when it is disbursed.