A co-signer is a person who agrees to sign a loan application with another person. This is often done in circumstances where the applicant's credit or income does not meet the lender's requirements for approval on its own, but adding a co-signer results in approval.
A co-signer assumes responsibility for loan payments should the primary borrower default on his payments. Late payments may also be listed on a co-signer's credit report, even if the co-signer did not know that the borrower was behind on payments. Co-signers may be responsible for late fees and collection costs should the loan become severely delinquent.