Pleroma is a free, federated social networking server built on open protocols. It is compatible with GNU Social, Mastodon, and many other ActivityPub and OStatus implementations.
The project consists of several components: Pleroma is the server implementation, and comes bundled with PleromaFE, the default frontend. Other useful utilities are also provided, such as an ActivityPub relay.
For a friendly introduction to Pleroma and the Fediverse, read What is Pleroma?