Domain Name System is a database of domain names that are translated into more computer-friendly numeric values. In other words, DNS translates the domain name that a human writes into a numeric value that computers can understand – an IP address. For example, when a user types google.com, it is translated into 126.96.36.199. This makes using the internet more convenient since people don’t have to memorize long numeric sequences. They can use meaningful text as a domain name instead.
There are, of course, rules about what kind of symbols can be used in domain names. These rules have been specified by The Internet Engineering Task Force (IETF) in Request for Comments (RFC, a publication from the Internet Society) 1035, 1123, 2181, and 5892. The RFC index can be found here.