They are legally called Consumer Reporting Agencies. They are for-profit business and are not government agencies, nor affiliated with any government. They are regulated mainly by the Fair Credit Reporting Act, and many other laws. They collect and keep consumers' information.
1. Equifax: The oldest one. Founded in 1899. Headquartered in Atlanta,GA. Largest credit bureau in the U.S. Publicly-traded on the stock market. 7,000 employees. Billions of dollars in revenue.
2. Experian: Founded in 1996. Headquartered in Dublin, Ireland. Second largest credit bureau in the U.S. Publicly traded on the stock market. 17,000 employees. Millions of dollars in revenue.
3. Transunion: Founded in 1968. Headquartered in Chicago, IL. Third largest credit bureau in the U.S. Publicly traded on the stock market. 4,100 employees. Over one billion dollars in revenue.
Many different credit bureaus exist in many other countries.
You are entitled by law to obtain one free credit report each year from each agency.