A participant is a fully interactive member of the video session who may or may not be publishing video  and audio feeds.
A publisher is a type of participant who publishes video and/or audio streams to other participants.
A viewer is a passive attendee of live streaming video. Viewers can still interact in real-time using signaling to chat, respond to polls, and ask questions.
A connected participant is a participant in a video session who uses Vonage Video API Audio Connector to connect to an ASR service or a 3rd party WebSocket server.