DNS is an acronym for Domain Name Server, and is the system used to translate word-based addresses of systems (such as WWW.EXAMPLE.COM) to the numerical IP (Internet Protocol) address of the computer or system that should be located at that address. All computers and systems on the Internet use addresses that look similar to: 126.96.36.199
When you use an alphanumeric address such as WWW.EXAMPLE.COM, your computer needs to understand what numerical IP addresses it needs to contact, and this is accomplished through DNS servers. The answer is delivered back to the requesting computer via the DNS listed for the domain name.
All domains have at least two DNS servers as seen through WHOIS lookups such as NS1.EXAMPLE.COM and NS2.EXAMPLE.COM, and your request for anything related to the domain name gets sent to one of these servers. In response, the DNS server sends back the IP address that you should contact. This works for the Web Site, Mail Servers, and anything else based on the domain name.