Lemmy is a decentralised discussion platform for communities, comparable to a link aggregator and forum. Users join communities, post links and text, vote and comment in threads, and use moderation tools, a public moderation log, private messages, RSS and Atom feeds, and blocking and filtering. Lemmy consists of many independently run servers that federate through ActivityPub, so users on one server can subscribe to communities on another. Anyone can host a server with Docker or Ansible.
Lemmy is open source under the AGPL-3.0 licence and written in Rust and TypeScript. No company controls it: development happens publicly on GitHub under LemmyNet and is funded by donations, so no central vendor holds user data. Where data resides depends on the instance operator. An organisation can run its own instance in an EU data centre, or pick a public instance after checking who runs it and where it is hosted. Content shared across servers is copied to other instances under their operators' rules. Self-hosting means handling updates, moderation and backups.