Each computer connected to the internet has its own numeric identifier – IP address, and all websites have their own names that are bound to this numeric combination. Therefore, domain registration provides ability to type text site name in browser’s address bar instead of remembering long set of numbers.
Information about domains and IP are stored in distributed network of DNS servers that allows to transform name into IP address by request and get access to it. Such system was invented to make it simpler for users to remember addresses that consist of words in written Roman or Cyrillic characters, separated by dots.
Each country has the first-level domain for providing users with the names of the second and third levels. If to register the site on ru domain, its address will look like “site.ru”, and it will be a second-level domain on the first level of Russian zone. Ua means that the site belongs to Ukrainian domain zone.
Initially there were a few first-level domains that were not related to geographic location. They reflected the type of site activity and distinguished websites by types. Org domain meant that the website content is not commercial, while com had the opposite meaning. Currently all zones are mixed up, and all comers can buy a domain that belongs to org, .net, com, pro, su, etc.
The vast majority of domains today have specific rules pointing at possible purposes for purchasing them and specifying who can buy them. There are more than ten domain zones that were not assigned to any specific country, such as .pro (professional), .travel (touristic), .net (organizations related to internet).
.Su domain has a separate position in this classification. This domain zone was reserved for the Soviet Union in 1990, but domains from this zone came to the market only a few years ago. Anyone can buy a domain in .su zone, there are no any limitations for registration. Currently there are many users who want to register a domain here thanks to the tendency of price decrease for this service. Besides this in the current zone it is possible to buy a domain in different national languages (.idn).
ICANN (Internet Corporation of Assigned Names and Numbers) is responsible for the general system administration. You can get more detailed information about each zone on their website.