Many organizations maintain their own newsgroup server. ISPs too, either host their own servers or rent one. There also various organizations selling you access to premium news servers.
Newsgroup servers maintain agreements amongst them to continuously synchronize themselves with latest posts. Hence, every new post which comes to newsgroups is copied over all the servers through the network. Any user accessing the newsgroup is connected to his nearest server and the copies of messages are then sent to her. This gives the newsgroups a distinct advantage of the speed of access.
Newsgroup discussions are organized by topics and the first name is indicative of what the discussion group is about. For example, soc. means social and cultural issues, biz. means business and alt. could be anything and so on.