Some of the types of loans that can be consolidated include Direct Subsidized Loans, Direct Unsubsidized Loans, Direct PLUS loans, FFEL program loans, and Perkins Loans. Generally you must wait until after you graduate or leave school to consolidate.
The decision to consolidate should be carefully considered. It may be convenient to make one single monthly payment, but you may end up paying more interest under consolidation if you extend your repayment period.
If you are consolidating to afford a smaller monthly payment (via an extended repayment period) do not allow your loans to default, which will delay or eliminate your ability to consolidate. Instead, ask your loan servicer about loan deferment.
Interest rates on consolidation loans will be unique to each borrower. It is calculated based on the weighted average of the loans included in the consolidation, rounded up to the nearest 1/8 of 1%.