WebRTC is a realtime communication engine; primarily works in the context of the browser. Though, there are native APIs as well.
It is not just a collection of API; it an innovative framework capable to define new protocols; suggest new gateways; etc. Remember, WebRTC isn’t a service in itself! Services can use WebRTC API to compete the world!
WebRTC is not a sub-part of browsers. Browsers implemented WebRTC native API to take advantage of its media engines and deliver media/data streaming using WebRTC-standard protocols.
It is not Google’s WebRTC! It is yours!!!
Think about WebRTC as a media streaming engine primarily developed for web developers.
Media Communication (RTCPeerConnection API)
Data Communication (RTCDataChannel/SCTP-datachannel API)
Media Capturing (getUserMedia API)
There is a separate draft/specification for Media Processing API as well.
WebAudio API has a separate specification as well; and can be used in WebRTC applications to process/initialize media streams.