What is a DNS server???
A Domain Name Server translates web domain names into numeric Internet Protocol (IP) addresses. Basically, it translates web addresses from a format you can understand into a format that the messaging protocols and hardware that route information around can understand.
The server itself is just one computer that helps route information around. Requests for webpages come in, it translates them into the exact IP address and starts to work out where to send the request to. Given the size of the internet it won’t store every single possible IP address, so if it doesn’t know the route to that address it’ll “ask around” other servers - think of it as a hierarchy, with the request being passed around until someone somewhere knows where to look.
DNS stands for Domain Name Server; it’s the server that controls the domain name you’re trying to reach.
I think. Oh well, someone will come along to correct me post-haste if I erred…
It’s not just for the web either. In general, all internet applications will be able to use either the IP address, or a domain name. If you have a Unix system, you can use the program nslookup to query domain name servers. This works by sending a short message to the server you specify asking about a given domain name. If you really want, I can go into detail about the exact format of the messages.
The server will either know the IP address of the domain name you are looking for, or know of a server to ask. Assuming that you haven’t told nslookup to disable recursion, each server will ask some other server through the same sort of messages, and you’ll get a message back with the answer. If you disable recursion, you just get a message back telling you a server that will get you closer to your answer (this really isn’t all that useful for most purposes).
Software like your web browser has these capabilities built in, simply using a DNS that you specify or one that your ISP provides for you. Thus, you don’t have to ever see IP addresses if you don’t want to.
Also, if you use ping with a flag of -a it will do the reverse, and find the domain name for the IP address you specify. This can be useful for things like checking out the IP addresses of suspected sockpuppets, although there’s also more advanced tracking software available.
Oh, and incidentally, it should technically be just DNS, not DNS server, but you probably say ATM machine and PIN number as well. 
OK
Thanks