top-level domain 
Part of the domain name that designates the country of origin or the category of organization (including government agency, commercial business and educational institution).
second-level domain 
Part of the domain name that designates the server name.
server 
Part of the URL address identifying the type of server. The best known is the Web server (www), which, as its name implies, hosts Web sites.
domain name 
Part of the URL address specifically designating the host. It must be registered with a domain name registrar.
communication protocol 
Part of the URL address specifying the language used to exchange data. The HTTP protocol, which allows the transfer of Web pages, is the best known.