Instant Messaging (IM)
Chat and instant messaging (IM) are short messages sent and read in real time, allowing you to converse more quickly and easily than email.
Chat and IM are generally used when both (or all) people are online, so your message can be read immediately. By comparison, an email message won't be seen until the recipient checks his email, meaning that chat and IM are better for quick messages and conversations.
Instant Messaging (IM) Application:
There are numerous chat and IM programs you can download, including Skype, ICQ, AOL Instant Messenger (AIM), and Yahoo! Messenger (all of these can also perform voice and video chat). In addition, there are many browser-based services that don't require downloading. For example, Facebook and Gmail allow you to chat with your contacts whenever you're signed in.
If you have a smartphone or mobile device, it's possible to use chat or IM when you're away from your computer. Popular mobile chat services include IMO and eBuddy.