A Centralised System is a system with one single point of contact for all parties involved, much like the Central bank, your retail bank, paypal and visa.
A Decentralised System is one in which no single point where the decision is made. Every node makes a decision for it’s own behaviour and the resulting system behaviour is the aggregate response.
A Distributed Networj essentially means that the processing is shared across multiple nodes, but the decisions may still be centralized and use complete system knowledge.
While some people have suggested Bitcoin is a Distributed Network, personally I reckon it is more like a Decentralised System, given how a number of sizable miners are in control of the network with their significant mining power.
Will there be a crytocurrency that reaches a perfect state of a Distributed Network, where there is truly no monopoly or oligopoly control over the network? We shall wait and see.