What is Clubhouse, invite-only audio chat app Elon Musk debuted on?

Elon Musk recently debuted on Clubhouse, an invite-only social networking app based on audio-chat. Users can listen in to conversations and interviews between people on various topics, similar to podcasts, but the conversations are live on the app in different 'rooms'. Founded in 2020 by Stanford University alumni Paul Davison and Rohan Seth, Clubhouse is reportedly valued at $1 billion.