Usenet is rather like e-mail only more so. Everyone connects to a server, and everyone sends their messages to a server, and in the mean time the servers exchange everyone's messages. When you log on, you receive all of the messages.
You can connect to News using Netscape or Outlook, which your Windows PC is likely to have already. News is a platform independent medium, however, which means that you can connect to it regardless of what sort of computer you use.
There are a great many newsgroups for people of all sexual persuasions, from the hardest sort of porn in alt.binaries.* to the tamest of cat discussions in rec.pets.cats.anecdotes
News is hierarchical: the different parts of a newsgroup name are separated by full stops. Names which begin uk.* are groups for people from the UK, but the Big 8 are frequented from people all over the world.
The two principal gay groups in the UK.* hierarchy are
Please note that personals are only welcome in the specific personals group.
uk.glb has a very helpful set of FAQs at www.ukglb.oaktree.co.uk/ including the extremely detailed FAQ2, which lists as many gay web resources as it can.