Domain name registration is the act of reserving a name on the Internet for a certain period, usually one year. It is important to know that this domain will remain yours for as long as you renew it and there is no way to purchase a domain name forever.
Domain name registration is necessary for a web site, an email or another web service. However you don't have to register always a new domain name. Many companies allow you to use subdomains from their domain names for a website or you can have an email with they primary domain (ex. yahoo.com, gmail.com).
The most important thing about a domain name registration is that it gives you personality and recognized identity. Once you register a domain name, the information about its owner is publicly available.