It's your address on World Wide Web.
As no two parties may ever hold the same domain name at the same time, it is a truly unique identifier of you or your company. It is how your customers will remember you and find you among the millions of other Web sites on the Internet.
Registering a Domain Name is the first and foremost step to start setting up an online identity.
The Internet Corporation for Assigned Names and Numbers (ICANN) is responsible for controlling IP (Internet Protocol) addresses and their associated domain names.
It also maintains a list of Domain Name Registrars. Domains are allotted on a first come first serve basis. Once a Domain Name has been registered, it is no longer available to anyone else until it expires automatically or is freed up by the registering authority.
Tips for selecting a Domain Name
It is always great if you have a Domain Name same as your organization's name.