Facebook Chat is a form of instant messaging, and Twitter can be thought of as a Web 2.0 instant messaging system.
Instant messaging is a set of communication technologies used for text-based communication between two or more participants over the Internet or other types of networks. Of importance is that online chat and instant messaging differ from other technologies such as email due to the perceived quasi-synchrony of the communications by the users.
Some systems permit messages to be sent to users not then 'logged on' (offline messages), thus removing some differences between IM and email (often done by sending the message to the associated email account).
Modern, Internet-wide, GUI-based messaging clients as they are known today, began to take off in the mid-1990s with Pow Wow, ICQ, and AOL Instant Messenger.
Similar functionality was offered by CU-See Me in 1992; though primarily an audio/video chat link, users could also send textual messages to each other. Meanwhile, other companies developed their own software; (Excite, MSN, Ubique, and Yahoo!
IM allows effective and efficient communication, allowing immediate receipt of acknowledgment or reply.
However IM is basically not necessarily supported by transaction control.
Many applications allow file transfers, although they are usually limited in the permissible file-size.
It is usually possible to save a text conversation for later reference.
In 2000, an open source application and open standards-based protocol called Jabber was launched.
The protocol was standardized under the name Extensible Messaging and Presence Protocol (XMPP).
Instant messaging systems tend to facilitate connections between specified known users (often using a contact list also known as a "buddy list" or "friend list").