How is the Usenet Organized?
The Usenet is home to hundreds of discussion groups called newsgroups that contain a cornucopia of unique subjects and topics. Whether you are looking to buy a house or want to learn how to repair your car, the Usenet has an answer and a seller for you. Because there are so many groups to choose from, navigating the Usenet can sometimes be a daunting task for new users. The early pioneers of the Usenet recognized this problem rather quickly and endeavored to enhance the Usenet experience by organizing the fast growing list of available newsgroups into 8 major categories, called The-Big-8.

Category Descriptions
comp.* These groups contain information and discussions about computer related hardware and software.
humanities.* These groups cover discussions and topics such as religion and spiritualism.
misc.* This category is reserved for any miscellaneous topics that don't fit into any other category.
news.* These groups cover Usenet Newsgroup related news and also world news and events.
rec.* Nascar fans, grab your steering wheels and start you engines. These groups include a vast variety of recreational topics guaranteed to appeal to everyone from the average hobbyist to the extreme sport enthusiast.
sci.* Scientists put on your lab coats. Theses groups cover any kind of scientific discussion on subjects ranging from the Big Bang theory to brain surgery.
soc.* Do you have questions about society politics or other cultures? These groups contain discussions and information about the many political and diverse cultures issues found worldwide.
talk.* The talk.* groups cover controversial topics and conversations not categorized in any other group.

For more information on Usenet Hierachies, please see this excellent guide on Wikipedia.