Mastodon is an open source SNS developed by 24-year-old German Eugen Rochko. The biggest feature is that there is more than one server called Instance, not one service controlled centrally by Twitter or Facebook.
Each instance is one that can be said as a small Twitter, and it is completed as an SNS by itself. However, there is a mechanism to follow users of other instances called remote follow, basically any instance you register, you can interact with Mastodon users all over the world.
You can check the instance list in Mastodon Instances.
