What is Mastodon, the social network users are leaving Twitter for?

Thousands of people who quit Twitter after Elon Musk's takeover have joined Mastodon, a free, open-source decentralised social media platform that allows users to share media and follow other accounts. Mastodon consists of independent, user-managed servers and users are based on one server but can interact freely with others. Unlike Twitter, Mastodon presents posts in chronological order.