DNS is a system that stores information associated with domain names in a distributed database on networks such as the Internet.

For example, when you type in a domain name like, for example, mozilla.org, you computer will automatically query the Domain Name System to find the actual IP address of the server you are trying to reach, for instance: 63.245.209.11.

You can see that query in more detail by opening a command window and typing in:
nslookup mozilla.org