Instant messenger
Instant messengers are a form of instant internet communication, alongside IRC and talkers (and chat in various games such as MMORPGs and MUDs).
Essentially, you load a program such as ICQ which then connects you to their server. You have whoever is on your friend's list to talk to. You then click on their name, write whatever, and hit SEND and then they get it instantly.
Whilst talkers were easier to use than this, instant messengers were seen as an improvement in that you could form your own groups, could send files and photos, did not have to enter your user name every time, and could effectively have it in a tiny window rather than dominating your computer.