Plus loans are either for parents of undergraduates or for graduate/professional students. These loans are different than other federal loans in that they require a check check and minimum score. It is possible to have a co-signer for both types of loans if the parent or student (depending on the type of loan) does not have a good enough score.
The yearly limit on both types of PLUS loans is equal to your cost of attendance minus any other financial aid you receive. The interest rate is dependent on what program your school works with, either 7.9% or 8.5%. There are also fees of up to 4% to get the loan. Graduate/ professional students also must have applied for their annual loan maximum eligibility under the Federal Subsidized and Unsubsidized Stafford Loan Program before applying for a Graduate/Professional PLUS loan. This a good things because the fees and interest rates are better for stafford loan than plus loans.
Just like a stafford loan, to receive a PLUS loan, the student/parent must fill out the FAFSA.