Synchronous one-to-one conversation via a service that lets interested others know when a user is online. Communication is more direct than with email and messages are typically very short. Although multiparty chats are usually possible, this feature is only seldomly used.
There's a large number of competing InstantMessaging protocols, each with their own, partially overlapping user bases:
And that's just to name the most important ones. The number of InstantMessengers, ie IM clients, is legion.
7 pages link to InstantMessaging:
