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The difference between a centralised government and a decentralised government is basically as their name implies. Centralised governments are ones in which legal authority and power is controlled by a singular political executive and all other arms of the government are subject to that executive.

Decentralised governments are governments in which high level decision making processes are distributed between the system and it isn't concentrated in a single person, body or place.

Just about every government to date is centralised and decentralised in one way or the other. There isn't really a government that exists today that is either of the two. The fact that each arm of every government is subject to the presidency, prime minister or whatever top government body is available makes it centralised, but the fact that democracy exists in most governments in its own way makes it decentralised.