When you co-sign for a loan, you are saying that if the person you are co-signing for doesn't pay the debt, you will. That loan will appear on both of your credit reports along with the payment history. If the other person doesn't pay, and the account becomes late, that late payment is going to show up on your credit report, and it's going to hurt your credit history too.
Be very careful about co-signing for someone. Make sure they understand just how important making those payments on time is to both their credit history and yours. Make sure they fully understand the favor that you are doing for them and the responsibility you are taking on for them. You are taking a risk for them and it could affect you negatively if they don't manage the debt well. Co-signing is something you should both take very seriously.