Domain
Domain is an identifier given to computers and networks on the Internet. It works like an address on the Internet.
Domain is indicated with a combination of alphabet characters and numbers (and a part of symbols). This was developed as a code name for IP address, because IP address, only an enumeration, is hard for human to deal with.
The system to relate a domain name to an IP address is called DNS, operated by linked DNS servers from all over the world.
Domain names have layered systems like addresses of real world. The most right part separated with "." is called top level domain (TLD), and second level domain, third level domain, and so on are following.
There are two kinds of TLD; the one called ccTLD, assigned to each country and the one called gTLD, assigned on the other basis than country. To block crash of domain names, an organization called ICANN is centrally controlling domain names, and organizations like NIC, registrars and registries entrusted by ICANN, are dealing with assigned management.
In most gTLD or ccTLD, the name of second level domain (the second part from right) can be determined by users, but in some ccTLD, second level domains show what kind of organization it is and users' name is in third level domain.
Japanese ccTLD (JP domains) consists of three kinds; organizational type JP domain names, whose second level indicates the kind of organization, geographical type JP domain names, whose second level or third level shows its autonomous body, and general purpose JP domain names, in which users register their preferable names on the second level.