Instant messaging (IM) technology is a type of online chat that offers real-time exchange of text messages between two or more people logged into a particular instant messaging (IM) service.
In order to set up an instant message, you have to add the usernames of the people you want to message to your contact list. When they log in and/or out of their IM software you are instantly alerted unless they have set themselves to invisible. Each system has its own method for blocking incoming and outgoing messages.