Domain Registration

A domain name is essentially a signpost on the Internet. Almost every website you've ever been to, and every email you've ever composed, has used a domain name in its address. People register domain names in order to 'stake a claim' to a particular name -- whether for business or personal reasons. Once a domain name is registered to a person or company, it is that person's to use exclusively as long as they continue to pay the yearly renewal fee and abide by the terms of use.
The 'Registry' is the system backend that is maintained by the operators of the TLD. Registrar's write new names to a central registry database, from which the authoritative root (essentially, a table of all domain names) is built. In the case of .com, .net and .org, the InterNIC runs the registry, and qualified registrars have shared access to it. In the case of many ccTLDs, the registry and registrar functions are combined within one entity.

World Wide Domain Name Registry