Voice over IP (VoIP) refers to real time voice traffic over Internet protocols. VoIP can be used on trunk lines between PBXs or other voice switches for reasons of efficiency or cost control. Likewise, VoIP can be used end-to-end to create voice services over computer network infrastructure. When VoIP is carried end-to-end, the resulting approach is called IP telephony. The marriage of voice and data communications on a unified network infrastructure is called converged networking.
VoIP can be provisioned with either the SIP or the H.323 family of protocols.
References:
http://transition.fcc.gov/voip/
http://www.cisco.com/en/US/products/ps6788/Products_Sub_Category_Home.html