Foucault himself likened his work to a 'toolkit' rather than a systematic or consistent body of thought. This is a very apt description. His work can be embraced by 'left' or 'right' (it is unlikely to appeal to many political moderates). In this sense, he has much in common with postmodernists.
In Discipline and Punish (1975), Foucault turned to the history of prisons, highlighting the 'panopticon' which had been developed by the Utilitarian philosopher Jeremy Bentham as a means of making prisoners believe that they were under constant surveillance. By extension, it could be argued that modern liberal democracies had become vast open-air prisons, in which the behaviour of citizens was subjected to constant regulation and interference by a state which was unable to use naked force as a means of day-to-day social control. In modern states, it was necessary for governments to give 'rational' explanations for techniques of control which had previously been wielded merely on the command of a sovereign (see page 381, Chapter 16). In other words, people had to be assured that they were being controlled for their own good. Foucault coined the term 'biopower' to denote these devices of modernity.
Thus, for Foucault, power is inherent in all social relationships, beginning with control over the definition of key terms like 'madness' and 'reason'
Foucault was chiefly concerned with power in its various manifestations. His early work on madness (in The History of Madness (1961)) was typical of this approach. The orthodox view at the time was that 'mad' people were genuinely dangerous to themselves and to others, and that although their treatment was sometimes difficult to square with civilised values the existence of asylums to keep them apart from society was a regrettable necessity. Foucault traced the development of the concept of madness in relation to its (supposed) opposite: 'reason'. From this perspective, 'mad' people could be regarded as those who acted in defiance of the ruling ideas in a given society; they must be treated inhumanely until they learned to obey the established rules of conduct.
Foucault was also fascinated by psychology; he had suffered from depression, partly perhaps because of his homosexuality. His first book (published in 1954) was in the field of psychology. However, after a succession of short-lived teaching posts he became a lecturer in philosophy at the University of Clermont-Ferrand in 1960. The following year he was awarded a doctorate, partly based on an examination of the concept of madness. After spending some years teaching in Tunisia, in 1968 Foucault was appointed Professor of Philosophy at a new university, Paris VIII, before moving to the College de France as Professor of the History of Systems of Thought. By this time he was increasingly drawn towards the United States, and lectured extensively at Berkeley, California. He also travelled to Iran, where he reported sympathetically on the 1979 revolution. He died of an AIDS-related illness in 1984.
Michel Foucault was born in Poitiers, western France, the son of a successful surgeon. Academically, he was a fairly late developer. In 1945 he failed the entrance examination for the prestigious Ecole Normale Superiore (ENS) in Paris. However, he managed to pass the following year and took a degree in philosophy (after once again failing at the first attempt). In 1950 he joined the French Communist Party, influenced by one of his teachers, the Marxist Louis Althusser. However, he was not a very active participant. This was fairly typical of the adult Foucault, who was too rebellious to be confined even by a subversive institution like the Communist Party.
Foucault developed different approaches for his different studies, but his work can be simplistically divided into 'early' Foucault, where he worked on the ways in which state power and discourses worked to constrain people, and 'later' Foucault (from the mid-1970s to his death from an AIDS-related illness in 1984), in which that idea of power as a 'thing' is broken down, and it is instead seen as a more fluid relation, a 'technique' which can be deployed.
Foucault rejected this view. For Foucault, people do not have a 'real' identity within themselves; that's just a way of talking about the self -- a discourse. An 'identity' is communicated to others in your interactions with them, but this is not a fixed thing within a person. It is a shifting, temporary construction.
We often talk about people as if they have particular attributes as 'things' inside themselves -- they have an identity, for example, and we believe that at the heart of a person there is a fixed and true identity or character (even if we're not sure that we know quite what that is, for a particular person). We assume that people have an inner essence -- qualities beneath the surface which determine who that person really 'is'. We also say that some people have (different levels of) power which means that they are more (or less) able to achieve what they want in their relationships with others, and society as a whole.
His interest in philosophical science and history led him to write extensively on the middle ages and the "archaelogy of knowledge." Shifting to a more geneological explanation of the transitions between major stages of human development led him to consider the causal effects of non-related causes upon the development of new thought.
Michel Foucault was a French philosopher, social critic, and historian whose vast influence extended across a broad array of disciplines, especially in the humanities and social sciences. He is perhaps best known for his ruminations on power, self identity, epistemology, and the evolution of systems of thought and meaning.