The decentralized social network known as the Fediverse is a collection of “instances”, or servers, independently operated but all speaking a common protocol to allow users to follow and share across different instances.
As well as social microblogging and photo/video sharing sites, the Fediverse includes other types of instances; all sorts of social services like event planning can have a Fediverse presence, and other sites like this blog can be Federated to share new content automatically.
The underlying technology is called ActivityPub; that’s the secret sauce that makes Ferated sites federate.
The key to the fediverse is that it’s decentralized. Although there are a number of large and popular instances like mastodon.social, that’s far from the whole story. Anyone can start up their own instance (if you don’t know how/want to run your own at home, there are plenty of turnkey hosting services who take care of all the tech for you), or even build their own software stack. One can find an instance catering to any interest or community.