Overview
ActivityPub is a W3C standard that provides a client-to-server API for creating, updating, and deleting content, as well as a server-to-server API for delivering notifications and content across different platforms (federation).
How it Works
It uses a 'mailbox' metaphor. Each user has an Inbox (for receiving messages from others) and an Outbox (for publishing their own activities). These activities are represented in JSON-LD format.
Impact
ActivityPub is the primary protocol powering the Fediverse, allowing users on Mastodon, PixelFed, and Lemmy to follow and interact with each other seamlessly.