SIP, short for Session Initiation Protocol is an IP telephony signalling protocol used to establish, modify and terminate VoIP telephone calls. SIP was developed by the IETF and published as RFC 3261.
SIP describes the communication needed to establish a phone call. The details are then further described in the SDP protocol.
SIP has taken the VoIP world by storm. The protocol resembles the HTTP protocol, is text based, and very open and flexible. It has therefore largely replaced the H.323 standard.