The following links provide definitions of their categories as well as further links to specific newsgroups and subcategories.
alt/
bionet/
bit/
biz/
clari/
comp/
ddn/
eye/
gnu/
hepnet/
humanities/
ieee/
info/
k12/
(Click on k12.lang/ for language specific newsgroups.)
misc/
news/
rec/
relcom/
rpi/
sci/
soc/
(Click on soc.culture for newsgroups that discuss cultures of other countries.)
talk/
u3b/
usenet@ieee/
vmsnet/