VOIP is like 'SIP'ping coffee

SIP is used great deal in VOIP communications and instant messaging.

Session initiation protocol (SIP) provides call processing functions like dialing a number, making a phone to rind, hearing ringback tones or busy signals etc.

eg:Microsoft Windows messenger uses SIP for instant messaging.

SIP is a peer-peer protocol. More on SIP can be found here.
Session Initiation Protocol - Wikipedia, the free encyclopedia