Learn all about SIP...

SIP is a signaling protocol used for locating remote users and establishing interactive communications. It is analagous to setting up a call on the telephone network, with two critical improvements.

1. It is internet-native, and therefore interoperates well with other protocols including, critically, future protocols (more at SIP RFCs and Specifications)
2. It separates session establishment and session description, so specifying who (or what) you would like to connect to is independent of how you would like to communicate. (Compare calling someone's voice vs fax line, where the choice of how to communicate dictates how you connect.)

SIP is the glue for a new set of existing applications, beyond just the obvious 'IP telephone' to include multimedia, mobility, IM and presence, e-commerce, web services, and many others.

To learn more,:
http://www.toyz.org/cgi-bin/sipwiki.cgi
http://www.cs.columbia.edu/sip/ (Maintained by Prof. Henning Schulzrinne MUST VISIT)
http://www.tech-invite.com/Ti-sip-WGs.html