Since private loans are not part of the Federal loan program, a FAFSA is generally not required. Private educational loans require credit approval (most will require students to apply with a co-signer). Federal loans may have more flexible repayment options, and other benefits, including deferment and cancelation clauses which may not be available through private loans.
We recommend students apply for financial aid and maximize their Federal loan borrowing before borrowing through private loans. Go to studentaid.ed.gov to view a chart showing a summary of the differences between Federal student loans and private loans.