The AT Protocol (atproto) is an Authenticated Transfer Protocol where user data is signed, enabling broadcast across services.

A Personal Data Server (PDS) hosts user data repos and signing keys, assigning handles and DIDs.

An AppView is an application in the Atmosphere, aggregating data from repos via PDSes.

A Relay aggregates data repos from PDSes, providing a stream of change events for AppViews.

Lexicon is a schema language for describing data records and APIs, similar to JSON-Schema and OpenAPI.

A data repo is a public dataset representing a user, containing JSON records and blobs, identified by a DID.

DIDs (Decentralized Identifiers) are universally unique identifiers for data repos, supporting did:web and did:plc.