They are two different things and can be considered contradictory to each other. Decentralised means that there is no central authority that can set policies and enforce them. All the decisions happen in locally on each server. Federated means that the servers have joined together in a union and appear as one.
The fediverse strives to be both decentralised and federated. Instead of one large company running everything there is many smaller servers who control their own little area. But these servers do work together so you can share posts across servers, this is the federated part. To an end user it therefore looks more like one single service. Each server administrator can set their own policies, have their own moderator teams, etc. They are also able to blacklist servers they do not want to interact with.
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