The co-signer enters an agreement to be responsible for the repayment of the loan if the borrower defaults. A lender will usually not go after the co-signer until the borrower defaults, but they can lawfully go after the co-signer at any time.
It has been stated by finance companies that in the case of a default most co-signers actually pay off the loans that they have co-signed for including the legal and late fees that end up being tacked on. Clearly this can be a large financial burden, and it can also reflect negatively on the co-signer's credit.
If you do agree to co-sign on a loan for someone, you can request that the financial institution agrees that it will refrain from collecting from you unless the primary borrower defaults. Also, make sure that your liability is limited to the unpaid principal and not any late or legal fees.
Upon co-signing you may have to brandish financial documents to the lender just as the primary borrower would have to.
Co-signing for a loan gives you the same legal responsibility for the repayment of the debt as the borrower. If there are late payments, this will affect your credit as well.
If you are asked to co-sign for someone, you may want to provide another option and suggest that they get a secured credit card. This way, they can build up their own credit history and not open themselves up to the possibility of taking on a debt too large, placing themselves, and you, in financial danger.