A Domain Registrar is a company that has been accredited by the Internet Corporation for Assigned Names and Numbers (ICANN), the domain name registration industry oversight body, to register and maintain domain name information for customers.
Any place that you can register a domain name must ultimately use a domain registrar. The use of domain registrars protects the public from multiple registrations of the same domain name and holds domain registrars accountable for keeping attached contact information current for a domain name. This protects domain registrants from third parties attempting to steal a domain name.
If you don't know who your current sponsoring registrar is, you can access information about your domain name by performing a Whois search.
You can find international domain name registries here.