The domain name system (DNS) maps internet domain names to the internet protocol (IP) network addresses they represent and enables websites to use names, rather than difficult-to-remember IP addresses.

Domain names give people a more intuitive way to access content or services than IP addresses: www.stormerhost.com instead of 206.19.49.149, for example. Most URLs are built around the domain name of the web server fielding the request: e.g., http://searchnetworking.techtarget.com/definition/DNS-attack. Web browsing and most other internet activity rely on DNS behind the scenes to quickly provide the information necessary to connect users to remote hosts.

How does DNS work?

DNS servers answer questions from both inside and outside their own domains. When a server receives a request from outside the domain for information about a name or address inside the domain, it provides the authoritative answer. When a server receives a request from inside its own domain for information about a name or address outside that domain, it passes the request out to another server — usually one managed by its internet service provider. If that server does not know the answer or the authoritative source for the answer, it will reach out to the DNS servers for the top-level domain — e.g., for all of .com or .edu. Then, it will pass the request down to the authoritative server for the specific domain — e.g., stormerhost.com or stkate.edu; the answer flows back along the same path.

source: http://searchnetworking.techtarget.com/definition/domain-name-system