I'm dedicatiing this page to a friend of mine who i met through the Archers website.  Lucky this page is for you and all others who don't want to ask their kids what something means!
The first thing I want to try to explain here is the difference between chat rooms, message boards, and instant messenger systems.  It's very easy to know which one you're using once you know the basic differences, too.

How to know you are in a
Chat Room:
A chat room usually has three spaces, one at the bottom where you type your message before hitting return to make it appear in the room.  The second, above this, is the largest part of the room where all the messages from people who are talking in that room appear, and the third is usually down one side (normally the right) of the main space, and it is where all the people who are talking are listed.  When you watch people talking in a chat room, their messages will appear on the screen as they finish typing them and press return, and you will have to do nothing to see them appear.  Often in a chat room, you can click with your mouse on a persons username and it will bring up another box so you can talk one to one with them without the others in the room being able to see your conversation.  (this one to one conversation is very much like an instant messenger window).  You will usually have a few options that you can do - depending on the chat room.  things like making your text colour different so you can see each other's messages more clearly and being able to click with your right mouse button on someone's name and seeing their profile (information they have chosen to share with you about themselves).

How to know you are using a
Message Board:
A message board works a bit differently from a chat room.  The first difference is that to see each new message that appears, you will need to refresh your page, even if someone else is using the board at the same time as you.  The second big difference is that you can't usually see who else is using the boards at the same time as you as there is no list down the side.  You do however use the board in the same way, by typing your message in the box at the bottom and pressing return, it will appear in the top part as a new message.  each message is usually split up a little more, and conversation is harder because you can't tell if anyone else is there at the same time as you.  There will often be a date or time stamp appear with the message you put up so that others will know when you said what you did, so they know how long it's been since a particular board was last used.

How to know you are using an Instant Messenger System:
An instant messenger system is very similar to using a chat room, except that normally to talk to someone using an instant messenger system, you will need to download a small piece of software, and you will usually know the person you are talking to and will have told them your user name, so they can contact you.  Like with both chat rooms and message boards, you normally type in a small box at the base of the screen, and the message will appear in the bigger box above.  like with a chat room this happens instantly with no refreshing needed, but you don't have a list of people who are talking on one side, as you will know the people you are talking to.  Also, you can normally only talk to a maximum of 3-5 people at a time, depending on the instant messenger you are using. 

There are all sorts of extra features which can be found in instant messengers and chat rooms, things like being able to talk and listen using speakers and microphones, being able to see the person you are talking to using a webcam (a small camera usually placed on the top of the computer so you can see each other).  Then there are smilies and abbreviations which I will talk about in a minute.  Hopefully now though, you have a basic understanding of what to expect when you use one of these facilities.
Abbreviations and their meanings:

Here is a list of abbreviations which you might find people using in chat rooms, instant messengers, or message boards.  I hope you find them useful.

Some of these abbreviations you may have already come across if you or your children use mobile phone text messaging.  Things like or8 have transfered across from there because of the need within texting to use as few charecters as possible, and because these then become easier for non typists to use in chatting on the internet too.

asl  or  a/s/l          Age, sex, location*
bf                        Boy friend
brb                      Be right back
fyi                       For your information
gf                       Girl friend
gigo                    Garbage in, garbage out
gmta                   Great minds think alike
imho                   In my humble opinion
imo                     In my opinion
lmao                    Laughing my ass off
lol                       Laughs out loud
lv                        Love
omg                    Oh my god
or8                     All right
ppl                      People
rli                       Real life interruption
rofl                     Rolls on floor laughing
wb                     Welcome back
wysiwyg            What you see is what you get

*(asking you for these details) you might reply by saying 23/F/England, however you do not NEED to answer this sort of question, do not be afraid to ignore a question like this, as many people (me included) often see it as impolite too.
If you come across any abbreviations which I've not included here which you think shoudl be, please let me know!