Well, this is not easy, because creating descriptions
of texts means understanding the text so you can decide what parts are important and which parts arent.
This is something that the searchengine builders have nightmares about.
Some solve it by just taking the first paragraph of text that is "normal" text (ie, not javascript or some other sh*t)
If you want true descriptions of a site you'll either have to do it yourself, or build one heck of a langua-intepreting routine that can describe what a text is about.
I'd opt for the "first paragraph" idea.