Stands for "Domain Name System." The primary purpose of DNS is to keep Web surfers sane. Without DNS, we would have to remember the IP address of every site we wanted to visit, instead of just the domain name. Can you imagine having to remember "220.127.116.11" instead of just "pc.net"? While I have some Computer Science friends who might prefer this, most people have an easier time remembering simple names.
The reason the Domain Name System is used is because Web sites are acutally located by their IP addresses. For example, when you type in "http://www.adobe.com," the computer doesn't immediately know that it should look for Adobe's Web site. Instead, it sends a request to the nearest DNS server, which finds the correct IP address for "adobe.com." Your computer then attempts to connect to the server with that IP number. DNS is just another one of the many features of the Internet that we take for granted.