default-master --> outputs the header (who's online, login, logout, pvt/home links) and footer html along with the context bar. This is usually the template I modify and the one I use as the value for the DefaultMaster parameter for my subsites
site-master --> outputs js necessary for xinha and other rtes
blank-master --> outputs the html head and body tags, content within <head></head>, links to css and other script files, meta tags, body tag attributes
Also, is there a reason why we have 3 master templates? It seems like an unnecessary complexity though I guess it does make things a bit cleaner.
Ok, good, so that's what they do, that's half of the answer... but (historically, big-picture, thinking about the graphic designer), what are they for?
There is a package called skin in CVS, but I don't know anything about it. It looks like it predates Lars' work on this, so probably not relevant. Other than that I don't know of any, sorry.