The first thing I looked at, I suppose, was https://activitypub.rocks/

There's lots of basic info on there, including links to the w3.org pages for ActivityPub:

https://www.w3.org/TR/activitypub/

and ActivityStreams:

https://www.w3.org/TR/activitystreams-core/

...the protocols that drive the whole "fediverse".

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