Domain Name System, or DNS for short, is the system which matches up domains and their IP address online. It is DNS which allows somebody to type in the domain name (www.google.com) into their web browser to access a website. It is a collected database which can be found on many servers online. The responsibility for the maintenance of this database falls upon the IANA (Internet Assigned Numbers Authority) and ICANN (Internet Corporation for Assigned Names and Numbers)
There are four different types of DNS record. The A record is the most important. It is the A record which holds the IP address for a domain. The MX record is for email addresses, the NS is the record of a nameserver, and TXT records are generally for certification, although they can hold a lot of unique text.