DNS (Domain Name Server) is a server that stores information about websites and addresses in a kind of distributed database on networks, such as the Internet.

You can build your own domain name server or dynamic DNS. There are also free DNS on the internet. Many websites offer different DNS services.

Most importantly, DNS provides an IP address for each host name, and lists the mail exchange servers accepting e-mail for each domain. You can set up your own DNS or purchase a DNS.

Definition from http://en.wikipedia.org/wiki/DNS