IM is a real-time communication method.
An instant messaging is an electronic notification that appears on your computer automatically. It can be used to notify you that an important message just arrived in your email, or someone wants to talk with you now, or something just happened that you wanted to know about.
IM allows easy collaboration.
You will become surprised by knowing the things that you can do by using IM facility.
Not like in communicating via email, here the parties know whether the peer is available. Most systems allow the user to set an online status or away message so peers get notified whenever the user is, available, busy away  from the computer.