What is the domain name system (DNS)?

The domain name system (DNS) enables each machine connected on the Internet to be recognized by a domain name. Every computer on the Internet has a unique IP (Internet protocol) address which consists of a string of numbers. Since the IP addresses can be difficult to remember, the DNS allows a familiar string of letters (the "domain name") to be used instead of the IP address. So instead of typing the IP address' arcane string of numbers you can type, for example, www.espn.com.

What does it mean to register a domain name?

The Internet domain name system (DNS) consists of a directory, organized hierarchically, of all the domain names and their corresponding computers registered to particular companies and persons using the Internet. When you register a domain name, it will be associated with the computer on the Internet you designate during the period the registration is in effect.
Note that the above description applies to domains that are registered and hosted. If the registrant elects to, a domain can instead be registered and parked. A parked domain name does not need a DNS affiliation.