Registering a domain allows you to point an easy name (ex. www.ultraonline.net) to your web hosting account. This also allows you to receive email. (ex. email@example.com)
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.