It is a W3C recommendation: https://www.w3.org/TR/activitypub/
Prominent implementations are Mastodon (https://joinmastodon.org/) and Pleroma (https://pleroma.social/). There are several others and mobile clients as well.
The network is federated, meaning that users of different "instances" (servers) can talk to each other. Instances are run by individuals, non-profits and corporations. They can have one user or many thousands. The funding models vary. Moderation is local to each instance.