Lemmy is free and open-source software for running self-hosted social news aggregation and discussion forums. The site has similar features to Reddit or Hacker News.

Lemmy instances are run by a large number of individuals or corporations, each with their own content moderation policies. Users on individual instances submit posts with links, text, or pictures to user-created boards called “communities”.

Communities are user created and local to each instance, however they may be posted to from other instances. Using a system of upvotes and downvotes users can influence what content appears at the top of the main feeds and of each community. Moderation is conducted by admins of each instance and moderators of specific communities.

Lemmy is part of the Fediverse which allows users from other platforms to interact with posts created by Lemmy users. ActivityPub is the protocol used to allow Lemmy instances to operate as a federated social network and allows users to interact with compatible platforms including Mastodon and PeerTube.