Federation in social networks

Seperate Mastodon instances are generally not run on the same server (but could be), each instance has it's own users and timeline and it can interact with any other Mastodon server out there. So if Alice is on the Instance example.com and Bob is using initrode.com then they can both interact with eachother as though there both shared the same server like on Twitter.

I find this vastly superior to the way Twitter works since they only have one set of rules which may not be compatible with some cultures in the world.