Your data, such as your posts, likes, and follows, needs to be stored somewhere. With traditional social media, your data is stored by the social media company whose services you've signed up for. If you ever want to stop using that company's services, you can do that—but you would have to leave that social network and lose your existing connections.
It doesn't have to be this way! An alternative model is how the internet itself works. Anyone can put up a website on the internet. You can choose from one of many companies to host your site (or even host it yourself), and you can always change your mind about this later. If you move to another hosting provider, your visitors won't even notice. No matter where your site's data is managed and stored, your visitors can find your site simply by typing the name of the website or by clicking a link.
We think social media should work the same way. When you register on Bluesky, by default we'll suggest that Bluesky will store your data.
But if you'd like to let another company store it, or even store it yourself, you can do that. You'll also be able to change your mind at any point, moving your data to another provider without losing any of your existing posts, likes, or follows. From your followers' perspective, your profile is always available at your handle—no matter where your information is actually stored, or how many times it has been moved.

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.