Definition:
DNS stand for Domain Name System.
The Domain Name System is like the phone book for the Internet. DNS maps the IP addresses (eg., 207.241.148.80) to human readable hostnames (eg., www.about.com). So DNS determines which physical server handles a domain's website traffic or email delivery.
Instead of one central directory, DNS is a distributed network of name servers. Each domain maintains their own authoritative name server instead of relying on one central directory. DNS services are included in every hosting plan so you don't have to worry about getting your own name server.