Subsidized loans are awarded on the basis of financial need. You won't be charged any interest before you begin repaying the loan because the federal government subsidizes the interest during this time.
Unsubsidized loans charge interest from the time the money is first disbursed until it is paid in full. The interest is capitalized, meaning that you pay interest on any interest that has already accrued. One way to minimize how much interest accrues is to pay the interest as it accumulates.