Simply, DNS is the internet mechanism that provides a specific computer readable address assigned to a specific human readable name. DNS is a distributed database, accessible worldwide, and is a marvel of cooperation. For example, google.com = 64.233.167.99. No matter where in the world you try that, you will get the proper response.

As you might imagine, a proper DNS system is important to your network. Here is a nice overview courtesy of Sprint.

DNS is used on private local area networks as well. A malfunctioning DNS system will cause excessive delays in network operations. While the concepts are easy to grasp, the implementation is sometimes counter-intuitive and not all system administators are comfortable setting up DNS services.