Domain name registrars are companies that have direct access to the registry of domain names. When you buy a domain name, you are actually purchasing it from a domain name registrar. A very well known domain registrar is GoDaddy, which has registered more domain names than any other company.
Domain name registrars host your domains by default, until the domain is are pointed towards another Name Server. Most domain name registrars sell domains for relatively the same price, with .com, .net and .org domains being sold for about ten dollars, and cheaper domains such as .info domains being sold for about one dollar. Most domain name registrars will offer a considerable discount for domain bundles.