From Forum Nokia Wiki

Instant messaging (IM) is fast way to communicate over the network such as internet between two or more device (computers, mobile phones) users sometimes called peers. You can send text messages from one device to another. The message is immediately displayed on the communicating person's screen (that's why it is called instant).

Following features are usually common for all IM technologies:

• User can see presence/absence(status) of the other user. The statuses are for example: DND (Do Not Disturb), Not Available, Online, Offline, etc.

• User can save conversation.

• User has list of other users he can communicate with. They are often called "buddies".

IM has usually client/server infrastructure:

• Central server, where user has to log in, that keeps track of the status of the user.

• IM client, where user writes the messages.

There are many IM technologies like ICQ, Jabber, MSN, Sametime, Skype. Most of them are nowadays bundled with voice services.

Having the internet connection they present very cheap way of communication between device users.