Real-time communication
IM differs from ordinary e-mail in the immediacy of the message and also makes a continued exchange simpler than sending e-mail back and forth. IM is less intrusive than a phone call but more immediate than email.
Set status
Unlike email, a user’s status can be represented in the IM service to notify people if they are available, busy, away, or offline. An optional message can also accompany this status indicator to inform users of their location, provide contact information, etc. Status indicators set expectations as to when a contact can reply to a message.