Alt.fan.pratchett (a.f.p.) is a usenet newsgroup that seems in no way related to Terry Pratchett, writer of the Discworld books. What is AFP then, you may ask. It's a newsgroup where people post messages about everything. The thing they have in common is that they like the works of Terry Pratchett, or are Terry Pratchett, or have heard about Terry Pratchett, or are completely lost but like it there anyway. This AFPost is a pretty good description of what goes on. For discusision about the books, you're best of in alt.books.pratchett (abp). Other Pratchett-related newsgroups are alt.talk.mended-drum, alt.fan.pratchett.announce (very usefull to keep up-to-date with tours, merchandise, meets etc), de.alt.fan.pratchett for the Germans and alt.fan.pratchett.bofh for when you are, obviously, a BOFH.
The difference between alt.fan.pratchett and alt.books.pratchett is quite big. A.f.p. is mainly a meeting place for the fans, to discuss anything that interests them. Because of the high traffic and different subjects, the "tags" were introduced: you might have wondered why all these [R], [I], [C], [F] etc are. The most-used ones are [R], meaning Relevant to Terry Pratchett and the stuff he writes, and [I], Irrelevant so basically everything else. The other tags, and all other important newsgroup-related stuff, is written in the different FAQs that all are at www.lspace.org/faqs/. Take your time to read them, as that will make a.f.p. a lot easier to understand, as well as avoid embarassing posts. Like all newsgroups, it is worthwile to lurk for a bit before getting more actively involved. When you decide to become an active poster, you are welcome, and there's usually people pointing you at the chocolate, cat with questionable parentage, comfy chair and alligator Sweetheart. Always remember that afp is a very friendly group, especially when you are nice too, and stick to netiquette. All about that in the FAQs, I don't think it's necesary for me to repeat what's in there.
Alt.books.pratchett is a lot quieter, as it is meant purely for the Relevant stuff. Tags aren't necesary because everything should be [R] anyway. When a topic drifts off beyond relevance, it is taken to a.f.p.
If there's anything you don't understand, you can always ask - but do show that you have first tried to find out by yourself. A.f.p. isn't there to do your homework, and in a lot of cases Google is your friend, both for web searches as for searching 20 years of usenet.
A.F.P. exists since 1992, and this is how I looked in that year...
AFPers don't just post messages, they also chat or meet - generally very friendly and welcoming to everyone who wants to have a good time. I have been to several official meets and one that wasn't official but we met anyway, and I do visit the IRC server of LSpace quite often. #afp is basically a fast version of AFP. Topics are wide-ranging, relevance to Pratchett even less, and even goldfish seem to have a slightly longer attention span.
Some personal a.f.p. stuff: I started posting with this, relurked and really started with this. I now have a lemon tree named after me...this was my suggestion, and this the happy news. Sanity the Lemontree is still doing well. Or even even better
I started using sanity on 19/2/2002 in this post. I was already using it on #afp since january.
Going from #afp to a.f.p.: the whole affordable stuff. Starting in this thread after "one of those nights on #afp", the occurences of affordable-prawns. In the same thread, this domain name was actually suggested...but it was onlyregistered about 2 months later, by me, after the affordable-domains really got going. Since 21 June I'm using the affordable-prawns address on a.f.p., and as my general a.f.p. domain. Plans, pictures and links still have to wait till I get myself round to do more about affordable-prawns.
One of the habits of afpers is to indicate drinks ending up on the keyboard, with *splutter*, *splorf* of C|N>K (for the non-geeky reader: Coffee through Nose on Keyboard). I managed to wet someone's keyboard only once.
Some must-have a.f.p. links:
L-Space 
(everything, including much about the newsgroups and Pratchett fandom in general)
The A-Files: The many faces and sounds of A.F.P.
And want to know which other groups I read? This is my .newsrc:
alt.books.pratchett alt.fan.alt-fan-pratchett alt.fan.hash-afp alt.fan.lisa-williams alt.fan.pratchett alt.fan.pratchett.announce alt.fan.pratchett.bofh alt.fan.pratchett.chess alt.fan.pratchett.chess-archive alt.fan.pratchett.old-farts alt.fan.sanity alt.humor.best-of-usenet kabelfoon.announce kabelfoon.discussie kabelfoon.help kabelfoon.os kabelfoon.vragen local.klijmij local.test local.test.moderated local.zandbak nl.comp.os.linux.announce nl.comp.os.linux.discussie nl.comp.virus nl.internet.misbruik nl.internet.misbruik.spam-signalering nl.kunst.cabaret nl.newsgroups nl.newsgroups.announce nl.onderwijs.algemeen nl.onderwijs.open-source nl.support.sysadmin nl.taal.citaten xs4all.announce xs4all.general xs4all.offtopic xs4all.test