VoIP is short for Voice over Internet Protocol. viop is the communication protocol by which internet telephony works.
Other frequently used synonymous to VoIP include IP telephony, voice over broadband also known as VoBB, broadband phone, Internet or broadband telephony.
Voice over IP has been setup and operated in many ways using both proprietary and open protocols and standards.
There are different technologies for implementing VoIP services starting from H-323, Session Initiation Protocol, IP Multimedia Subsystem and Realtime Transport Protocol.
Some of the well known proprietary implementation of the VOIP is Skype or Vonage.