Instant messaging, or IM, is the name given to communicating with someone else, via a keyboard, when that other person is online at the same time, as you can see in the screenshot below. In this respect it differs from emailing, tweeting or texting, where the other person may pick up your message later -- when perhaps you are offline.
When IM first appeared on the scene it achieved notoriety because of the way some people conducted themselves in chat rooms in Yahoo!, MSN and other providers. In fact, in the teen rooms especially, but also in 'adult' rooms, some people would use IM for writing sexually explicit content. I found that the sensible rooms were sometimes quite useful, as there were rooms for all different interests, but because it was always a bit hit and miss as to who was going to be online at the same time, these were never as good as today's communities and the ability to 'friend' or 'follow' someone.
There was also the ever-present danger that, because IM is text-based, you could never be certain that the person you were speaking to really was who they purported to be. Not that that particular danger has gone away, as the recent Facebook murder story sadly illustrates. However, there are ways of teaching youngsters about this sort of thing, as Dughall McCormick illustrated in Computers in Classrooms last April.
Today, many of the original chat rooms have disappeared, and those that remain seem to be overrun by spam. However, IM has entered the mainstream, insofar as anything Web 2.0 may be termed 'mainstream'. You will find it in applications such as Skype, streaming video, Facebook, Google Wave.
The reason is that IM is a brilliant way of communicating with someone else instantly. You can quickly share an opinion, ask for a file, give them a useful URL. Used sensibly, IM can provide a rich extra dimension of communication whilst engaged in another activity such as taking part in a discussion or watching a video.
Like anything else, such as text messaging, just because IM has been abused, does not mean it should be abandoned all together.