What does the intelligent folders work?

I m testing the Firefox 3 (i love it :D ) ... but, i have some doubts... one is related to the intelligent folders, how do i set a "inteligent" folder?, or, in other words: what does it mean?.

I have read that a "inteligent folder" is a search in bookmarks and historial that can be set as a folder, BUT, i have seen that Firefox 3 incorporates 2 default "inteligent folders", one is te most visited sites and the other is the "added recently" sites... so, it means that we can expand the utility of the "inteligent folders", we can make a dinamic folder that changes frequently... but, HOW????... because, according to what i read, we can make only a folder with especific search and is not a dinamic folder....

Thanks to all!... and i think that help should be better in the firefox,