A domain registrar is a an organization or company that manages registrations for domain names. Registrars keep track of who is registering a domain, and update the appropriate databases. Registrars charge a fee for this service.

Some domain top-level-domains, like .COM, have many registrars. Others, particularly country registrars, have only one.

If you're looking for an unbiased list of COM registrars, try Registrar Judge.

See more info at Wikipedia.