What are the fundamental beliefs of Michel Foucault and those that changed the thinking of philosophers of his time?

Michel Foucault was a French philosopher who explored three important subjects that had been mostly ignored in philosophy until then: madness, sexuality, and prison. He also did some important work in redefining journalism.
Much of Foucault's early work is related to madness, on which he also did his thesis. A key quote from his works on madness is the following: "Si le personnage medical peut cerner la folie, ce n'est pas qu'il la connaisse, c'est qu'il la maitrise." (If a medical figure can understand madness, it is not that he knows it, it is that he controls it.) Several key works of his are on madness, dreams, and mental disorders.
Foucault's work on sexuality determines that different eras used sex and related elements to define and "codify" individuals—including the understanding of a person or their behavior. Foucault's argument is that sexuality should cease to be central to determining the nature of an individual.
One of Foucault's major works is on how restrictive freedom of expression is, and he goes on to analyze what determines what can be said and what is inappropriate to publicly say or privately think. This work is closely related to prisons, in which he retains some interest. Foucault thought the prison world was void of all possibility of expression and demonstrates the terrible state in which imprisoned individuals live without public recognition of these conditions.
On journalism, Foucault considered that it was the reporter's duty to make "diagnostics" on the present, to understand the "today," and to explain why and how events came about.
These four elements of Foucault's work shaped the future, particularly the latter two. For example, the influence of Foucault's ideas about sexuality can be seen in today's sexual identity movements. Foucault's ideas about conditions in prisons and expressions can be linked to the abolition of capital punishment and the establishment of better prisoner conditions in France. In journalism, one can notice a clear shift in reporting as journalists put Foucault's ideas into practice by taking the responsibility of explaining rather than simply reporting events, as was originally the definition of a journalist and the role of the press.