In their own words:
Its somewhat like Twitter, but owned by the community. While Twitter limits you to 140 characters, Mastodon allows you 500."Mastodon isn’t just a website, it is a federation—think Star Trek. Thousands of independent communities running Mastodon form a coherent network, where while every planet is different, being part of one is being part of the whole."
One of the best things about Mastodon is its community led protection against abuse, harassment and trolls, something which Twitter is either unable to or too slow to react to.
With over 2.2 million people on Mastodon, its already becoming a social media network to jump into.