Subsidized student loans are Federal loans that are available to undergrad students that don't accrue any interest on the loan as long as the student is in school.
Unsubsidized student loans are Federal loans as well but the student must complete a FAFSA to be eligible for the loan.
With an Unsubsidized student loan the student is responsible for any interest that accrue's on the loan even when the student is still in school.