ActivityPub

Users are actors and actors have inboxes (messages to read) and outboxes (messages to send)

• You can POST to someone's inbox to send them a message (server-to-server / federation only... this is federation!)
• You can GET from your inbox to read your latest messages (client-to-server; this is like reading your social network stream)
• You can POST to your outbox to send messages to the world (client-to-server)
• You can GET from someone's outbox to see what messages they've posted (or at least the ones you're authorized to see). (client-to-server and/or server-to-server)

Actors post activities to their outbox, or sometimes objects, like a note, but that just gets wrapped up in a Create, for example. This is done via client to server interactions. A client will POST to the actor's outbox, discoverable via their profile.

The federation part of Activity happens via server to server interactions. Activities get posted to actors' inboxes.

Delivery is usually triggered by, for example:

• an Activity being created in an actor's outbox with their Followers Collection as the recipient.
• an Activity being created in an actor's outbox with directly addressed recipients.
• an Activity being created in an actors's outbox with user-curated collections as recipients.
• an Activity being created in an actor's outbox or inbox which references another object.

Also you can read about the different Objects and Activities here: https://www.w3.org/ns/activitystreams