Foucault's ideas of power revolve around his ideas of discourse.  For Foucault, power is derived through discourse.  Those discourses that are accepted by the main body of society are the hegemonic discourses.  Therefore the groups that promote these hegemonic discourses become, in a Grammscian sense, the hegemonic groups in society.  These discourses become hegemonic because they are promoted, and eventually accepted, as the Truth.  The truth, in modern societies, is derived from knowledge, especially that from the human sciences.  So for Foucault, knowledge and power are directly linked to each other.
Clearly then, the groups in a society that can control the knowledge, and how it is used, will have the ability to  determine, at least to an extent, what the truth is.  From this they will then be able to exercise at least some power within the society.
To Foucault, power was not socially or structurally static.  Rather, power was historically specific to the particular society at a particular time, and could only be understood in this context.  To understand how power worked in a particular society at a particular time, it was necessary to understand the techniques used to exercise the power.
Yet Foucault did not see power as being completely dominating.  He did not see power as being based on a centralized agency, rather he saw power as being diffused throughout society, through complex social networks.  This diffusion of power allowed for resistance to power within a society.  Foucault argued that whenever power was exercised there was likely to be resistance to it.  This resistance was based on discourse.  By creating a critical knowledge of a hegemonic discourse, those resisting it could then claim a new truth of their own.  To be successful, the resistance would be more effective when used against a technique of power, rather than attacking the general power or discourse.  This new truth, if accepted, could then become the new hegemonic discourse, which would then attract new resistance to it.  In this way, power, discourse, and truth were all fluid concepts.  They were never static or permanent, but were always subject to resistance and change.


We can use Foucault's theory to determine the basis and place of power in a society.
Consider this:

Foucault argues that centralized or state power, that is, power exerted from above, is not the real or main power within a society.  That power is diffused throughout society by networks of power.
To Foucault, this leaves us with nothing more than the illusion of power, in that we think that our national government, or state apparatus, and politicians is where power is to be found.

Yet the reality is often the opposite.  Politics, and the exercise of power within the nation state, is usually seen by most people in society to be centred around the national political structures, such as federal governments.  In this way the idea of sovereign power, or the central government with considerable powers, can be seen to be the hegemonic discourse in Western political thought.

This tends to undermine Foucault's theory.
If most people accept the ususal discourse on sovereign power, this takes on the mantle of being the Truth, and therefore becomes the basis for people's actions and decisions.
From this it can be argued that far from power being diffused throughout society as Foucault argues, rather, much power is in fact to be found in the institutions of the sovereign power.
This is not necessarily because the sovereign power is or has this power intrinsically, rather, because people believe that it does, and therefore naturally defer to the sovereign power.
In effect the sovereign power has its level of power mainly from the fact that most people accept its discourse as truth.  This saves the sovereign power from having to regularly use force to exercise its power over the society.
This does not mean that the sovereign power has all or most of the power in a society, and that local or individual power is effectively non-existent.
Rather, because people believe that the sovereign power is where the society's main power is to be found, they are either reluctant to go against it because it appears so powerful, or because they actually turn to it for assistance and leadership.
This can be seen in contemporary Western politics were many groups, rather than directly opposing the government, actually try to change policies by direct interaction with the government.
Thereby effectively operating within the existing sovereign power's rules, in effect recognizing and legitimating the power of the sovereign power, and its accompanying discourse.

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