Once Usenet became available to the general public in the 1980s, the amount of discussions increased very quickly. Because of the broad number of topics being discussed, there needed to be some sort of organizational scheme. Today, that scheme is based on what is called a hierarchy. For any Usenet group, the hierarchy is denoted by the first part of the name, which is usually a three- or four-letter abbreviation for the full name of the hierarchy.
There are 8 main hierarchies and these are often given to as the Big 8. These Big 8 hierarchies are the hardest to get a new group started in and, for that reason, they are transported by the vast majority of Usenet service providers. These newsgroups also tend to be those with the least amount of spam, the most diligent moderation and the highest number of expert participants. There are usually enough experts involved in any given newsgroup that those who make inaccurate posts will quickly find themselves admonished for it and the issues they muddled will be clarified by other users.
The newsgroups, because they are so numerous, tend to have very specific names. Using these names is like using any other index. Each part of the name helps clarify what the subject is about. For example, comp.linux is a computer newsgroup that deals with the Linux operating system. Finding the newsgroup you want is usually no more difficult than searching for key terms and then sorting through the results to find the closest match. There is usually more than one match for any given search simply because there are such a huge number of newsgroups available, even within the exclusive Big 8 hierarchies.
There are hierarchies that are focused on specific regions and on specific institutions, there are even hierarchies that focus on specific companies. These are usually specialty hierarchies and they may not be carried on your server. There should be some, however, that are regional and specialized in other ways. Most often, you'll find anything you want to talk about in the Big 8. There are additional common hierarchies, as well, such as the bionet hierarchy, which specifically focuses on topics of interest to biologists. Each of the hierarchies has thousands of groups and, even without the expanded list, there is plenty of information to be found on just about every topic, most of it quite reliable.