There are some good info about the interest rates of student loans, and I want to clarify a few points.
subsidized Federal Stafford: 6.8% fixed (maximum of $8,500 as of 2008)
unsubsidized Federal Stafford: 6.8% fixed
Federal Direct Graduate PLUS: 7.9% fixed (requires credit check, but score isn't considered*)
Federal Family Education Graduate PLUS: 8.5% fixed (requires credit check, but score isn't considered*)
private student loan: interest rate is variable (changes over time based on Prime Rate or LIBOR) and is based on your credit score (higher score = lower rate)
For a subsidized Federal Stafford loan, the government pays the interest on your loan while you are in school. For other types of loans listed above, you are responsible for the interest while you're in school.
The difference between Federal Direct and Federal Family Education is the source of the borrowed money. The money will come directly from the government under a Federal Direct loan, whereas it will come from banks (Chase, Citi, Sallie Mae, etc.) under a Federal Family Education loan. Your school chooses which loan its students will receive.
Private student loans come from banks, require a credit check (higher score yields lower interest rate), and offer a variable interest rate. This variable interest rate could go up or down, depending on the Prime Rate or LIBOR that you and the bank agreed to.
* "A qualified Graduate PLUS Loan borrower does not have an adverse credit history (defined in regulations as being 90 days or more delinquent on any debt, or having a credit report that shows default, discharge, foreclosure, repossession, tax lien, wage garnishment or write-off of a Title IV debt during the five years preceding the date of the credit report). Note that Graduate and Professional Student PLUS loans do not use any kind of a debt-to-income ratio or FICO score, unlike private education loans."
source:
http://www.gradloans.com/graduate-plus-loan/