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Plain English Explanation of Foucault's Archeological Method?

I really want to understand Foucault's arguments, but I'm having trouble understanding his methodology, and every explanation of archeology I've found is super convoluted.

I get that it's an alternative to traditional methods of uncovering history, but what evidence is used? And what exactly is the discursive level? Hopefully this isn't a dumb question.

Edit: Thank you. It probably isn't easy to distil a concept like this, so I really appreciate the answers given. I'll definitely be looking more into Foucault to try to grasp the complexities of his work, and with this confusion sorted out, I feel more confident in doing so.
gpt
Archeology is the study of systems of thought by attending to the way in which they are governed by rules which the subjects of those rules are not aware of (they are below or beyond the consciousness of subjects).  The rules, for instance, constrain conceptual possibilities (what they can think and therefore do).  The subject matter of Archeology is, as you suggest, discourse - literally just how people talk at a given time.  So, the Archeologist selects a subject at a particular time, studies how people talk about that thing at that particular time and tries to induce out of that the rules for talking about that particular thing.

The assumption is that the historian can discern how we think (even unknowingly) about [x] by observing how we talk about [x]. 

So, in *History of Madness*, we look at how people talk about insanity at different times and try to discern the conceptual rules for talking about insanity.  We will quickly discover that insanity is different at different times - the rules for talking about it are contingent to a certain discursive situation.  So the Archeologist goes about comparing these different conceptual sets by looking at how people talk about things.  It focuses on the conceptual consequences of different ways of talking about what you otherwise might think was 'the same thing.'

In general this disrupts the kind of historiography that tries to see history as progressive and continuous in some clearly rational way.  This is why, for Foucault, science is such an interesting object of study since it purports to be progressive with respect to the acquisition of knowledge.

ETA - I've tried to say this in a way that avoids too much Foucaultian talk.  Some serious Foucaultians will be (perhaps rightly) offended by that minimization.