The nationwide three
Equifax, Experian, and TransUnion are the three nationwide credit reporting agencies. They each maintain their own file on you, sourced from the companies you have accounts with, so the three reports are related but not identical.
Secondary reporting agencies
Beyond the big three are specialty and secondary consumer reporting agencies — such as LexisNexis Risk Solutions and Innovis — that maintain their own consumer files. Information can flow between these and the primary bureaus.
Because of that, reviewing and, where appropriate, disputing information at secondary agencies can help keep a corrected item from reappearing later through another source.