Newsgroups are collections of articles, which are organized into threads. The various newsgroups are organized by their topic, which is reflected in their names. The first word in a newsgroup's name, e.g. alt.food.chocolate, identifies the type of group. Usually they are related to each other, but not always. Subscribing Subscribing to a newsgroup allows you to keep track of a select number of groups, without having to search the full group list to see if they have been updated since you last read them.
Posting is the act of placing a message onto a newsgroup.