A while ago a colleague of mine asked if anyone could explain to him what value they found in Michel Foucault. He wanted a jargon-free quote-less explanation.
What follows is my attempt.
Admittedly my reading of Foucault is unorthodox, but I take him to be a kind of perverse functionalist. Because this is supposed to be jargon-free, I will carefully define that term and some related ones.
Functionalism and Sociology
Functionalism in sociology looks to explain the existence of an institution or a practice with reference to how it perpetuates the society in which it is found. Emile Durkheim used this method to explain the existence of crime: For him, crime revealed points of stress or antagonism in a society. This allowed agents within it to act effectively to redress the underlying problems that crime revealed. Crime was a kind of social immune system.
The problem with functionalist social theory is that it's tremendously teleological. It makes social theorists into predictors of the future, which they have never been good at. Sometimes functionalism even seems to suggest, wrongly, that we live in the best of all possible worlds: when even crime, suicide, and other horrible things ultimately contribute to social stability, one may come to see them as inevitable and natural rather than as problems in their own right that might be averted. Unsurprisingly, functionalism is also very bad at explaining social change.
The structuralist view
Structuralist sociology sought to correct these problems by bringing in new ideas about the origins of social institutions and practices. Many of these new ideas were utterly wrong, but at least they weren't the just-so stories of functionalism.
Structuralists were significantly influenced by Hegel, and they tended to explain social phenomena with reference to the Hegelian theory of the mind, which that asserted that we think in pairs of binary oppositions, and that we map these theses and antitheses onto the world around us. The thesis-antithesis-synthesis dialectic gives rise to social change.
My view of structuralism is that it sometimes seems correct, but that it's also a dead end, albeit for a different reason: Structuralism is impossible to falsify, making it dubious as a scientific theory. It was against this background that Foucault did his work.
Foucault never called himself a post-structuralist, but I think the name is apt, and it is certainly better than calling him a postmodernist. He respectfully disavowed structuralism, and, I would argue, he offered a more modest, resolutely non-teleological functionalism: It is not a whole society that perpetuates itself through various institutions and practices. Rather, specific institutions perpetuate themselves, often quite opportunistically and at the expense of others in society.
The role of institutions
Institutions gain power in a society by making persuasive knowledge claims, and often by inventing whole new domains of knowledge, to which they or their agents purportedly provide access. The prison, and the study of penology, legitimate themselves and gain currency by asserting the existence not merely of crime, which was nothing new, but of criminality, a social status that is distinct from simply the act of performing a crime: A criminal has a criminal mind, and perhaps also a criminal body, and we need to know what to do with these people.
Now, this is perhaps sometimes quite true, especially for hardened or lifelong criminals. But surely claims about criminality have been oversold, and the creation of the criminal as a specific social type - and in effect as a second class of citizen - tends perversely to perpetuate the prison. We could likely do with much less incarceration than we have, but we have been convinced that some people simply are criminals, and that we can find no other place for them. They are handed over to the experts for treatment in the institutions that they run for the purpose. And this may not be to the benefit of the society itself; one gets the strong impression in reading Foucault that the prison and everything about it is a kind of parasite on the rest of society.
A truth about Foucault that can't be uttered in the academy is that this very process, this production of knowledge through institutional power, has also taken place in the modern university. Identity studies are indeed suffused with (badly understood) Foucault, but a proper understanding of Foucault might well indict these studies themselves: They have invented fields of knowledge, they control the access to these fields, and they are perpetuating themselves perhaps at the expense of the larger society around them.
Institutions and access to knowledge
As with penology, some of the knowledge to which identity studies stakes a claim may actually be valuable, but a good Foucauldean would ask some tough questions anyway. It is in the nature of institutions to perpetuate themselves - that's not original to Foucault - but his new move, as I see it, is to observe that institutions in the modern world tend to perpetuate themselves by controlling access to knowledge, and commonly they control access to knowledge about some very important and very personal topics: who we are, what our minds are like, what our bodies are like, what our souls are like. Starting first with the last of these, modernity has handed these subjects over to experts.
That may be a good thing, but it ain't necessarily so. We may want to have some say about who we are as well. Foucault's famous closing to The Order of Things imagined a world in which the study of Man might cease to exist in any sense at all. It is no longer so difficult to imagine the way this might happen. Man will be fractured into a near infinite array of warring classes and identities, each with its cadre of warring experts. And with each division, the knowledge possessed by any of them will grow smaller and smaller, less and less universal, less and less subject to the refinement that comes from testing against the ideas and experiences that arise in other modes of life.