Voice over IP (VoIP) defines a way to carry voice calls over an IP network including the digitization and packetization of the voice streams. IP Telephony utilizes the VoIP standards to create a telephony system where higher level features such as advanced call routing, voice mail, contact centers, etc., can be utilized.
Session Initiation Protocol (SIP) is a peer-to-peer, multimedia signaling protocol developed in the IETF. SIP is ASCII-based, resembling HTTP, and reuses existing IP protocols (DNS, SDP, etc.) to provide media setup and teardown. Since its first publication in 1999, SIP has generated a high level of interest in the VoIP industry, and many people believe that SIP will become the de facto standard protocol for future voice networks.