Newsgroups are now organized into a hierarchy of subjects defined by those extensions. A good discussion of this is found at (vide
"The great renaming" section):http://encyclopedia.thefreedictionary.com/usenet
When you use OE to bring in the list of groups origianlly, these are maintained by OE, and can be updated from wherever you are getting the original list (most likely your ISP) in your news account.
Many newsgroups can be found and read in Google by using its Group selection on the main page.