When I talk about twtxt with friends, usually that question emerges.
What’s the difference between Federated, Distributed, and Decentralized?… And also the difference between platforms, systems, services, and such.
I remember we discussed that a few months ago.

I think you should have a post in your blog or similar as a future reference.

I would define things these terms like this:

• Decentralised systems are those in control of the data and its locality.
  
• Distributed systems are systems with participating nodes in a network that cooperate in decisions with data locality over the whole network.
  
• Federation enables systems and users to communicate across boundaries using identities.
  

The key differences come down to data locality and control.