DNS - domain name system. This system helps to find a real server by virtual site name. The system is realized by the hierarchy of DNS servers, each of them holds information about specific domains and may delegate such role to lower stage servers. Let's take, for example, site www.host-tracker.com. The upper domain is .com, so the request is made to the server, which is responsible for all .com sites. It knows where each site is located, or at least, where is another server, which knows for sure. If you get "Server not found" message, that means no record exists on the DNS server for the site name you've requested.
Each time you enter an address into the browser - a DNS request is made, in order to find where the site is located. To make this process faster, DNS data can be saved into a cache. If you own a site and decide to change its name for some reason, or if you have just created the new one - you should keep in mind that DNS cache needs some time to be updated globally. Usually, it happens within couple of hours, but it can take up to 48 hours. That's why sometimes a newly-hosted site can not be visible for some time.