Simply put, they keep a record of who has given you credit, when they gave you credit, how much credit you are given and whether or not you paid it back on time. When you want to obtain credit cards, loans, financing for a car or home, leases, apartments and sometimes even employment, the lender or bank will check your credit to see your financial history.

Credit Bureaus are paid by the people who request your credit file.
Credit Bureaus have no legal power over you. Banks, police or the government does not run them; so don't be intimidated by them. They are the Credit Bureaus because they own large computer systems capable of storing credit information on everyone in the United States. However, because of the tremendous amounts of information on their computers, their method of storing information is very basic and ridden with many errors. Since the bureaus have made so many errors in the past, all Federal Laws regarding credit information are very much in your favor.

Information from americanloansearch.com