Instant Messaging (IM) is a form of real-time direct text-based chatting communication in push mode between two or more people using personal computers or other devices, along with shared clients. The user's text is conveyed over a network, such as the Internet. More advanced instant messaging software clients also allow enhanced modes of communication, such as live voice or video calling and inclusion of links to media.
Instant messaging (IM) falls under the umbrella term online chat, since it is also text-based, bi-directionally exchanged, and happens in real-time. IM is distinct from chat in that IM is based on clients that facilitate connections between specified known users (often using a contact list, buddy list, or friend list). Online 'chat' includes web-based applications that allow communication between (often directly addressed, but anonymous) users in a multi-user environment.

#from http://en.wikipedia.org