Usenet is a worldwide distributed discussion system where the users express their opinions about thousands of topics. In each server there are countless groups to discuss about a topic by sending messages very similar to email.
You could say Usenet is a hybrid between web forums and email, but in this case forum sections are called newsgroups and messages are called articles.
Because there are so many newsgroups, Usenet newsreaders give the possibility to subscribe to the groups that interest the user. This way the user can read the new messages comfortably.