Explanation of WP filters and wp_head()? (Black-boxing aggrss and lastRSS)

I’m probably missing something obvious, but I can’t seem to find an explanation of the concepts behind WP filters. I see lots of calls in the WP code to functions that mess with filters but without a clue I can’t make much sense of them. Are the basic ideas documented anywhere besides this bare list?

This comes up because I’m trying to figure out what goes on behind the scenes in the infamous wp_head() call required by all plugins. That call evaluates to apply_filters('wp_head', '') at which point I lose the trail.

And that question in turn stems from the thing I’m really trying to do, which is clone the bare minimum environment needed to run aggrss and lastRSS in a non-WP setting. I’m happily using them in my WP blog and would like to do so in another context as well. It looks at first glance like they don’t really use much WP infrastructure, but then I run into the classic undefined function: get() error that points to wp_head(). Details about what I’m trying to do are in my comments on Per Soederlind’s aggrss page. Suggestions welcome!