PeerTube is a free software video-sharing platform that uses ActivityPub to build a network of many different instances. As the code is open-source, anyone can host a PeerTube instance themselves, for instance via Yunohost but many associations and individuals already propose instances where one can just register and upload videos. The use of ActivityPub allows the PeerTube network to be part of the wider Fediverse.
It is developed by the french non-profit organization Framasoft.
You can watch an introductory video about PeerTube here
PeerTube is written mostly in TypeScript, HTML, CSS, and SQL. It provides a javascript frontend to access and read the videos.
The main novelty of PeerTube is that it uses WebTorrent to enable peer-to-peer streaming of videos. This means that, while watching a video, you may receive/send parts of the stream from/to other users that would be watching it at the same time. This is especially useful to reduce the load on the servers and enable small instances to serve content to many users, notably when using live-streaming, which started being beta-tested recently.
Unlike Youtube, PeerTube does not contain ads or tracking and is therefore much more privacy friendly.
Each instance has its own rules for content publishing, meaning that you can choose an instance that fits your needs, or if none does, you can host your own.
Because it uses ActivityPub, you can follow PeerTube channels and accounts as well as post comments on videos using any account on the Fediverse (e.g. your microblogging account on Mastodon or Pleroma).
Because PeerTube is decentralized, moderation must be performed manually by the admins or the users on their channels, which means some extra work. Furthermore, since anybody can host their own instance, admins must be careful who they federate with.
A list of instances that do not moderate their content or promote or host content deemed illegal in several countries (notably France) is available on Peertube Isolation. It is highly recommended to admins to ues this list to block these bad actors and prevent their content from appearing on your federated page.
If you are on an instance that enabled peer-to-peer, your IP address will be visible to any user watching the video at the same time as you unless you disable the feature in the preferences. Though this should almost always be a non-issue, it has been deemed a problem for institutions in French education regarding students privacy and GDPR, which is why all these instances chose to disable peer-to-peer.