Credit bureaus collect consumer credit information, then resell it to other businesses in the form of a credit report.
In a nutshell, they track consumers' habits about paying back borrowed money. This information is important to lenders, who use it it determine things such as how much interest you will be charged on a loan.
There many credit bureaus in the United States, but most people are familiar with the big three: Equifax, Experian, and TransUnion.
Want to find out what your credit score is? You can request a free annual credit report at AnnualCreditReport.com
USA.gov also has useful information, including directions on how to obtain your credit report, what to do if you need to fix your report, and info about any recent data breaches that might affect you.