There are basically 5 types of loans you can get.
The school will look at the cost of attendance, consider your family EFC (and possibly that of your parents, even if you are older, have your own family, etc.). From there they will calculate your need, less any scholarships, and typically start with:
Federal Direct Subsidized (no interest accrual during school): I believe the limit on that is 8,000.
Federal Direct Unsubsidized (interest accrues during school): I believe the limit on that is 12,500.
Most schools usually give both out at the maximum level. Depending on how much money you still need, schools will give out:
Grad PLUS loans, sometimes offered through the Federal loan program, more often offered through a lender. The rates on these loans are usually pretty high, at or around 8%. Be careful with these.
Sometimes the school will give out their own loan or scholarship, depending on the school.
In rare cases you will need to go through a private lender to get extra financial aid. These are risky, so be careful and study up on them. Your credit history will be a big factor in determining your interest rate, and there are a lot of different repayment plans, so pay attention.