Lemmy is a selfhosted, federated social link aggregation and discussion forum. It consists of many different communities which are focused on different topics. Users can post text, links or images and discuss it with others. Voting helps to bring the most interesting items to the top. There are strong moderation tools to keep out spam and trolls. All this is completely free and open, not controlled by any company. This means that there is no advertising, tracking, or secret algorithms.
Federation is a form of decentralization. Instead of a single central service that everyone uses, there are multiple services that any number of people can use.
A Lemmy website can operate alone. Just like a traditional website, people sign up on it, post messages, upload pictures and talk to each other. Unlike a traditional website, Lemmy instances can interoperate, letting their users communicate with each other; just like you can send an email from your Gmail account to someone from Outlook, Fastmail, Proton Mail, or any other email provider, as long as you know their email address, you can mention or message anyone on any website using their address.
Lemmy uses a standardized, open protocol to implement federation which is called ActivityPub. Any software that likewise implements federation via ActivityPub can seamlessly communicate with Lemmy, just like Lemmy instances communicate with one another.
The fediverse ("federated universe") is the name for all instances that can communicate with each other over ActivityPub and the World Wide Web. That includes all Lemmy servers, but also other implementations:
In practical terms: Imagine if you could follow a Facebook group from your Reddit account and comment on its posts without leaving your account. If Facebook and Reddit were federated services that used the same protocol, that would be possible. With a Lemmy account, you can communicate with any other compatible instance, even if it is not running on Lemmy. All that is necessary is that the software support the same subset of the ActivityPub protocol.
Unlike proprietary services, anyone has the complete freedom to run, examine, inspect, copy, modify, distribute, and reuse the Lemmy source code. Just like how users of Lemmy can choose their service provider, you as an individual are free to contribute features to Lemmy or publish a modified version of Lemmy that includes different features. These modified versions, also known as software forks, are required to also uphold the same freedoms as the original Lemmy project. Because Lemmy is libre software that respects your freedom, personalizations are not only allowed but encouraged.
You can contribute to this documentation in the git repository.
This page is adapted from Mastodon documentation under CC BY-SA 4.0.