Abstractioneer recently posted the shortest guide to Atom and Atompub I’ve ever seen:
Essential Atom and AtomPub in 30 seconds
Atom is this: You have a bunch of things, or sometimes just one thing. They always have unique ids, they have timestamps, and tell you who created/is responsible for them. Oh yeah, if you can, please provide a short snippet of text describing each thing.
AtomPub is how to discover, create, delete, and edit those things.
Everything else is optional and/or extensions.
Now that’s concise.