Mastodon is free and open-source software for running self-hosted social networking services. It has microblogging features similar to the Twitter service, which are offered by a large number of independently run Mastodon nodes (technically known as instances), each with its own code of conduct, terms of service, privacy options, and moderation policies.
Each user is a member of a specific Mastodon instance (also called a server), which can interoperate as a federated social network, allowing users on different nodes to interact with each other. This is intended to give users the flexibility to select a server whose policies they prefer, but keep access to a larger social network. Mastodon is also part of the Fediverse ensemble of server platforms, which use shared protocols allowing users to also interact with users on other compatible platforms, such as PeerTube and Friendica.