The Fediverse (federated universe) refers to a network of independent servers providing a common service. User can choose to sign up with one of thousands of instances all running the same interoperable software. The servers are run by different people with different policies. This is not the same as an Open Service because it is not necessarily permissionless and not free of censorship either.
ActivityPub is the name of the common software that enables these separate instances to interface with each other. Sometimes different Fediverse protocols are interoperable. For instance, a Pleroma user can follow a user on a Mastodon instance.
The fediverse idea has attracted a variety of people. Many Mastodon instances take a harsher censorship stance than Twitter and maintain blocklists of instances that don't enact safe-space policies. Blocklists are a great starting place when choosing an instance. The reasons for blocking are almost always unjustified.
the-federation.info is a site that tracks statistics about various fediverse projects.
- Mastodon - decentralized twitter
- Pleroma - decentralized twitter
- GNU Social - decentralized twitter
- Peertube - decentralized videosharing