Mastodon is a micro-blogging service and social network platform similar to Twitter. Unlike Twitter, Mastodon is a decentralized service based on the World Wide Web Consortium's ActivityPub protocol. Users may select any Mastodon server they want, or even host their own. The Mastodon website contains a server selection wizard that allows users to find a server with content moderation policies that are to their liking. The 500-character "toots" are listed in strict chronological order without any optimization to insert advertising or promote particular users. Each toot can be set to one of four tiers of visibility, with the most permissive appearing everywhere and the least only being visible to a specific user. In addition to Mastodon's web interface, which works in any recent browser, numerous client applications are available for Windows, macOS, Linux, Android, and iOS. The Mastodon server is a free software with source code available on GitHub.