VoIP stands for Voice over IP.
VoIP is an umbrella term for all voice and video real time communications over the internet. In that sense, WebRTC is a subset of VoIP.
Looking to learn more about WebRTC?
Check my WebRTC training courses
The WebRTC Glossary is an ongoing project where users can learn more about WebRTC related terms. It is maintained by Tsahi Levent-Levi of BlogGeek.me.