Welcome to Mastodon 

One of Mastodon’s fundamental differences to Twitter is federation. To bring that word into context, the United States of America are a federation. In a more technical context: E-mail is federation. It means that users are spread throughout different, independent communities, yet remain unified in their ability to interact with each other. You can send an e-mail from GMail to Outlook, from Outlook to someone’s private e-mail inbox. Mastodon’s federation is similar: users from different sites (let’s call them “instances”) establish connections between these sites by following each other and sending each other messages like on any other social network.

Source: Welcome to Mastodon – Eugen Rochko – Medium

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