I have been on Mastodon since January and just found Lemmy a few weeks back. The initial confusion is completely normal, but they both run on the exact same underlying architecture: the Fediverse.

If you understand how email works, you already understand how Lemmy and Mastodon work. You can have a @gmail.com account and send messages to a @yahoo.com account because both servers speak the same protocol. Lemmy operates exactly like this, but for forum posts instead of emails.

Here is the breakdown of how it works.

1. What are these called? They are called instances (or sometimes "nodes" or "servers"). An instance is simply a single, independent server running the Lemmy software. lemmy.ml is an instance. lemmy.world is an instance.

2. Does an account on one instance exist on others? No. Your account obamacares is permanently tied to the database of the server where you registered (lemmy.ml). You do not exist on lemmy.world. However, because of how the network operates, your lemmy.ml account can interact with users and communities on lemmy.world seamlessly.

3. Do posts and comments exist on other instances? Yes, through a process called federation. If a user on lemmy.world subscribes to a community hosted on lemmy.ml, their server will reach out to your server and request a copy of the posts and comments. The data is duplicated and cached across the network wherever there is active interest.

4. If banned from one instance, are you banned from all? No. Bans are strictly local. If the admin of lemmy.ml bans you, your account on that server is dead. However, you can simply go to a different instance, register a new account, and immediately resume interacting with the wider network. The only exception is defederation. If a specific instance becomes a hub for spam or toxic behaviour, the admins of other instances can block that entire server, severing the connection completely.

5. What is the relationship between the instances? The relationship is a shared language. They communicate using an open-source protocol called ActivityPub. It is the standard that allows an independent server in Germany to exchange posts, votes, and comments with an independent server in Brazil. Mastodon uses the exact same protocol, which is why a Mastodon user can follow and reply to a Lemmy community directly from their microblogging feed.

6. How are they stored and served? There is no blockchain, no peer-to-peer torrent magic, and no decentralised "pool." It is brutally simple: an instance is just a standard computer (usually a rented Linux VPS) running a web server and a PostgreSQL database. It is maintained and paid for by whoever set it up--usually an individual admin or a small group of volunteers.

7. How much privacy is there? Assume absolutely zero privacy. Private messages (DMs) on Lemmy and Mastodon are not end-to-end encrypted. Structurally, a DM is just a standard post with its visibility flag set to "direct" rather than "public." This means the database administrator of your instance-- and the database administrator of the instance the person you are messaging belongs to --can read your messages in plain text by querying their database. Never use Fediverse DMs for sensitive information.

8. Are deleted posts gone for good? Not reliably. When you delete a post on lemmy.ml, your server deletes it locally and broadcasts an ActivityPub Delete request to every other instance that previously copied it. Well-maintained instances will receive that request and honour it, erasing their local copy. However, you have zero control over those remote servers. If a server is offline, misconfigured, or run by an admin who refuses to process delete requests, your comment will linger on their hardware permanently. Treat everything you post to the Fediverse as indelible.