@doriane

PDS (all user posts are stored here. anyone can connect to a PDS endpoint and get a live feed of a user's posts. or a combined feed of all user's posts)
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Relay (connects to every PDS's combined feed. you can connect to an endpoint and get a giant combined combined feed)
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AppView (Connects to a relay. Puts every post in a big database.)
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Client (Web page or phone app that shows posts from an AppView)

I'm happy to explain terms but ATP is so complicated, I can't fit it all in one pos