DNS is a distributed database. It maps hierarchically-organized keys to various sets of record values. Its most common uses include mapping hostnames to IP addresses and back and finding an organization's mail servers.
Name-address mapping was originally done via an /etc/hosts-style file that was distributed to all hosts. DNS was created to replace this mechanism, providing better scalability and easier updates. It was also created to be extensible: it can readily accomodates new types of data associated with keys in the database.