But first we must briefly define centralisation.
Centralization is a psycho-social propensity towards a dominance system based on a fixed hierarchy. See also politics, religion, sociobiology.
Postmodernism, which literally means "after what is now", incorporates information theory and nonlinear dynamics. (See also: information dynamics) The most topical idea in postmodernism is the rejection of all metanarratives. By rejecting the meta-narrative, postmodernism rejects centralization.
Centralization is a solution to the fundamental philosophical problem of "human welfare", economy, and efficiency. - That the ideal would be a central organizing device that, by being central, can relate all problems to each other and in a sense "solve for all variables". Centralization believes that important sociological decisions belong in the hands of a few, and assumes "a priori" that they are capable of managing these decisions.
Decentralization thinks about sociological dynamics in terms of information channels and cybernetics. It rejects the assumption that hierarchies are in-themselves "optimal" for any purpose, but considers them, on the contrary, arbitrary structures. But decentralization is not anti-hierarchy, rather it is only anti- central and fixed hierarchy. Nor is decentralization pro-anarchy, but is rather the antithesis of anarchy, insofar as aristocracy is the ultimate result of anarchy.
Decentralization puts the "power" in the hands of information. It believes that power structures emerge out of social dynamics, and those social dynamics are exercised through information channels such as media technologies. It believes, furthermore, that information is necessary to solve problems, whether they be social, economic, or what have you, and that it is beneficial to those who are effected by a problem to have clean and effective information channels to the problematic, efficient means to solve the problems, and effective information channels to communicate their situation to other organizations who may be in different situations, but are nonetheless dependant on the state of this organization.
Decentralization is the belief that the interdependencies of these organizations cannot be simplified into a hierarchical structure or "solved" via a "top-down" approach. The solutions must, on the contrary, be solved at each point from each perspective, and the solutions transmitted to the other points and re-evaluated continuously.
A fixed hierarchical structure would inhibit the healthy dynamics of such a system. It would, for instance, create longer information channels, susceptible to more noise and separated further from the problematic, in such instances where a short feedback loop would be more effective.
In the end, centralization believes that information channels should have a fixed, rigid, and hierarchical structure, whereas decentralization (and postmodernists) believe that information channels are by nature, and should be, fluid and adaptive.
This is not to say that decentralization prefers a meshwork, i.e. transactions between members of roughly equal power. Rather, decentralization prefers something like a self-organizing map; a spatially dissipative structure. (See: self-organized criticality.) And a system where power relations are formed through play. (read: "How the Leopard Changed it's Spots") In such systems, hierarchies are actually quite common but never fully solidify, and there is never a "grand hierarchy".
Decentralists do not believe that information channels shouldn't exist and therefore that power relations should not exist, but rather that there should be more of them, and that they should be cleaner and more direct. Whereas centralization seeks one, all encompassing information channel, decentralization seeks many simultaneous information channels, and rejects the idea they should be or even can be merged into one.
See also: emergence.