You're right -- his work is difficult to read. And it's not your fault. He doesn't write very clearly; it might be said that he also didn't think very clearly. However, he had some interesting ideas. If you want a quick summary, start with this awesome video: youtube.com/watch?v=BBJTeNTZtGU.
Basically, Foucault wanted to challenge the holders of power in modern society, as well as their Enlightenment-based conventions, values and attitudes about science, medicine, insanity, prisons, and sexuality. His method was to describe the historical origins of these conventions, values and attitudes (though he didn't have much care for historical accuracy).
2 examples of important ideas (or families of ideas) from Foucault:
- Medicine and madness
Foucault thought that (1) Western civilization's treatment of the insane hadn't improved since the Renaissance, (2) during the Enlightenment, their treatment became less enlightened, and (3) modern people think about madness in the wrong way (i.e. as a medical issue).
In History of Madness (1961), Foucault claims that during Renaissance, insane people were engaged with (at least at the corners of society). He says they were not regarded as people suffering from illnesses, but rather as people with mysterious wisdom. After making various historical claims about how perspectives and practices regarding the insane changed, he suggests that modern attitudes toward the insane are not more enlightened than those of the Renaissance and earlier times. He criticizes modern people for ostracizing madmen (physically and socially), and blames this phenomenon on the fact that madness is viewed as a medical illness in the modern age. He misses some key historical facts, but he poses some interesting challenges to modern assumptions about insanity and the place of madmen in society.
In The Birth of the Clinic (1963), Foucault gives us the idea of the "medical gaze:" a set of attitudes that became popular in the late 1700s (at the dawn of modern empirical medicine), attitudes that separate bodies and persons. Foucault thought these attitudes were dehumanizing.
He also offers broader philosophical criticisms of notions of scientific progress and scientific realism. In The Order of Things (1966), for example, he claims that the supposed objective progress of medical science is really better described as a series of epistemological ruptures (not really analogous to Kuhnian paradigm shifts, but the comparison is tempting).
- Power & the penal system
When Foucault talks about "power" -- as he does incessantly -- he's talking about disciplinary control over people. Foucault is especially interested in punishment because he sees it as an example of how social and governmental institutions in post-Enlightenment society are set up to protect the bourgeoise from revolt. Once again, he's trying to challenge Enlightenment ideas and values that he thinks underpin modern social and political institutions.
In the case of the penal system, Foucault is interested in the move from public punishments of a criminal's body to attempts to control the criminal's mind/soul behind closed doors. When you do bad things to someone behind closed doors, you're less likely to spark revolt.