Atprotocol.

Mastodon and activitypub is very much like social media implemented in the form of public email lists implemented over HTTP (push massages between servers).

Bluesky uses content addressing (think magnet links for torrents, but for linking to post data) and pull mechanism with notifications, plus public shared archive nodes (CDN-like servers).

The main difference is that you’re much more independent of individual servers and can easily move your account around, since your account ID is tied to a cryptographic key belonging to your account which you bring with you along with your post archive, and your handle can be based on a domain name you own (also works as a verification mechanism you don’t have to pay extra for).