What are some differences between distributed and de-centralized services?

I am unable to come up with fundamental differences between the two. I could only come up with examples:

• BitTorrent is a distributed p2p system, but ToR is decentralize开发者_StackOverflow中文版d.
• Web services are decentralized, but I cannot think of a distributed web-service. (Maybe Diaspora?)

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distributed systems can be in two type .. centralized and decentralized . Centralized Distributed Systems ,as client-server system and Multitired Application . Decentralized Distributed Systems not contains server ,such as peer to peer communication .

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If architecturally there is a single point of failure within a system, it is not a distributed system. A decentralized system could be a hierarchical structure where there is still a notion of the root of the hierarchy, a point of failure. A traffic round-about is a distributed version of a centralized system where traffic-lights (in traditional sense) is used.