A catergory of Internet-based "chat" technologies that typically enable rapid text communication between two people. A key feature of IM is the ability to have a "buddy list" of friends or peers, and see at a glance which friends are available for chat. This capability is known as "presence".

IM conversations can occur PC-to-PC, phone-to-phone, or PC-to-phone.

Several separate IM networks exist on the Internet, including Yahoo!, ICQ, AOL, and MSN.

The IM interface on a phone can be implemented as a proprietary client that connects to one or more specific IM network(s).

There is also a phone-industry standard for IM created by the OMA standards group called Wireless Village. Phones with Wireless Village can connect with a standard Wireless Village server on the carrier's network, which can, in turn, connect with one or more IM networks. This emilinates the need for software on the phone tied to specific IM networks.

IM clients can also be implemented as BREW or Java applications, either built-in or downloadable.

Some carriers also allow basic IM to be accessed via WAP or SMS, although this method has limitations in both speed and functionality.