A credit report is a consumer's financial report card. Lenders report their history with you to the credit bureaus, and the credit reports are the summary. It lists the types of credit you use, how long your accounts have been open, and whether you pay your bills on time (among other factors).
Credit card companies, banks, mortgage companies, auto loan and insurance companies, landlords and employers use credit reports to check on your credit history before deciding on doing business with you. Why? They know that if you were responsible in the past, you are likely to be responsible in the future (and vice versa).