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, the user might type in gmail.com and this will be translated into a number, such as 22.214.171.124. This way, people don’t have to memorize IP addresses and computers can use simpler symbol combinations than alphabetic strings (think of Chinese characters or special characters like õ in Estonian or å in Swedish).
There are, of course, rules about what kind of symbols can or cannot be used in the 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.