How it works
Anyone can run a server of Mastodon. Each server hosts individual user accounts, the content they produce, and the content they subscribe to.
Each user account has a globally unique name (e.g. @firstname.lastname@example.org), consisting of the local username (@user), and the domain name of the server it is on (example.com).
Users can follow each other, regardless of where they’re hosted — when a local user follows a user from a different server, the server subscribes to that user’s updates for the first time.
Why is that cool?
Servers are run independently by different people and organizations. They can apply wildly different moderation policies, so you can find or make one that fits your taste perfectly. If one server goes bankrupt or starts acting unethically, the network persists so you never have to worry about migrating your friends and audience to a yet another platform again.