Lemmy is a federated [1][2.1][2.2] forum [2.1][2.2].
Lemmy is a federated platform for news aggregagtion / discussion.
A link aggregator and forum for the fediverse.
Lemmy is similar to sites like Reddit, Lobste.rs, or Hacker News: you subscribe to forums you're interested in, post links and discussions, then vote, and comment on them. Behind the scenes, it is very different; anyone can easily run a server, and all these servers are federated (think email), and connected to the same universe, called the Fediverse.