What is the 'Recently Taged' in FF mean? What do you use it for?

Could you explain this to me please? I have the latest version of FF 3.0.1, I believe and it showed up when the new version did also the ‘most visited’, why do we need these?

I am fairly new to FF and don’t ‘play’ with the settings to much so would really like some help knowing what these are for and also if there is an easy way to organize the bookmarks I use often. How do you ‘categorize’ your bookmarks? Thank you for any help you can give.

Sorry if this is a somewhat ‘dumb’ question to ask.