Mastodon is a free, open-source social media platform that allows users to create and join decentralized networks called "instances." Each instance operates independently but can interact with others, enabling users to share posts, follow each other, and engage in discussions across different communities. Unlike traditional social media platforms, Mastodon emphasizes user control and privacy. It is designed to be ad-free and does not collect personal data for profit. Users can customize their experience and choose which instance to join based on their interests, fostering a diverse range of online communities.