In 1993 the U.S. Department of Commerce, in conjunction with several public and private entities, created InterNIC to maintain a central database that contains all the registered domain names and the associated IP addresses in the U.S. (other countries maintain their own NICs (Network Information Centers) -- there is a link below that discusses Canada's system, for example).
Network Solutions, a member of InterNIC, was chosen to administer and maintain the growing number of Internet domain names and IP addresses.
This central database is copied to Top Level Domain (TLD) servers around the world and creates the primary routing tables used by every computer that connects to the Internet.
[1] The competition created by the shared registration system enables end users to choose from many registrars offering a range of related services at varying prices.
Only the designated registrar may modify or delete information about domain names in a central registry database.
Barriers to entry into the bulk registrar industry are high for new companies without an existing customer base.
[5] If a domain registration expires, irrespective of the reason, it can be difficult, expensive, or impossible for the original owner to get it back.
After the expiration date, the domain status often passes through several management phases, often for a period of months; usually it does not simply become generally available.