MobileMe iDisk Folders
Backup is used by the MobileMe Backup application. I don't recommend using it.  
Groups is for a now defunct Groups feature that they used to have.  
Library is used for MobileMe's syncing feature - primarily for Preferences and Bookmarks.  
Sites is used for webpages or anything you want shared at http://homepage.mac.com/username/. Note: PHP files, CGI files, etc. don't work. It only works with static content.
Software contains free software provided by Apple to MobileMe customers (eg. the Backup application) - this doesn't affect your disk usage.
Web is a folder used by iWeb - otherwise it's nearly identical to the Sites folder but is available at http://web.me.com/username