(also known as ) is when, instead of one big site, you have lots of smaller independent sites that talk to each other.

Here's a short article about why federated services are a good idea, and how all of us use them all the time (even if we don't realise it):

The idea of federated social media like Mastodon, Diaspora, PeerTube etc is to give users more control over their social network and personal data. Federation and decentralisation (as well as distributed) are not the same. ;)
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Mastodon is a microblogging site that federates with most instances on the Fediverse.