What Are ActivityPub and the Fediverse in Mastodon?

ActivityPub is the open, standardized protocol Mastodon uses to implement federation, and the fediverse ("federated universe") is the name for all websites that can communicate with each other over ActivityPub and the World Wide Web. In practice, this means a Mastodon server can talk not only to other Mastodon servers but also to other software that implements ActivityPub, so your account is not confined to one platform or one provider.

How the pieces fit together

Mastodon is a microblogging service: you publish small updates to a stream on your profile, with optional media such as pictures, audio, video, or polls, and you can follow friends and discover new ones.

Federation is the mechanism that connects these services. Instead of one central service that everyone uses, there are multiple services that any number of people can use. The Mastodon documentation places these on a spectrum:

A Mastodon website can operate alone, like a traditional website where people sign up, post messages, upload pictures, and talk to each other. Unlike a traditional website, Mastodon websites can interoperate, letting their users communicate across sites. The documentation's own comparison is email: you can send a message from Gmail to someone on Outlook, Fastmail, Protonmail, or any other provider as long as you know their address. Likewise, you can mention or message anyone on any website using their address.

ActivityPub is what makes that possible. It is the standardized, open protocol Mastodon uses to implement federation, and any software that implements federation via ActivityPub can communicate with Mastodon seamlessly — just as Mastodon websites communicate with one another.

What counts as the fediverse

The fediverse includes all Mastodon servers, but also other implementations that speak ActivityPub. The documentation names several:

• Pleroma — a modular microblogging engine
• Pixelfed — a federated image sharing platform for sharing and consuming media posts
• Misskey — microblogging alongside a customizable dashboard
• PeerTube — upload videos to channels
• Plume — publish longer-form articles
• And many more, including individual and personal websites

Because the fediverse has no brand of its own, you will more often hear "follow me on Mastodon" than "follow me on the fediverse," though the documentation notes the latter is technically more correct.

What this changes for you

Two practical consequences follow from the documentation:

• Choice of service provider and policy. Because Mastodon is just software that can power any website, the site you sign up on is a choice, not a given. Different providers can set different policies.
• Reach beyond Mastodon. Your posts and follows are not limited to Mastodon software. If a service implements ActivityPub, it can interoperate with your Mastodon account.

If you want to understand Mastodon's design before choosing a server, the key takeaway is that federation via ActivityPub is what separates it from centralized networks: the network is the collection of interoperating sites, not a single company's platform.