A folder holds emails - you can copy or move to it. Filter to it based on criteria etc.
A VIRTUAL folder looks at other folder for criteria
I use a lot of graphics so I have a virtual folder named ART that looks for certain color groups that I have set up. You can set one up to look at all your flagged and or parked messages, or Todos and to answer (first set up something that it can recognize like a color group)
I have VFs to watch all my inboxes, another for sents and yet another for trash (it makes emptying the trash a lot easier)
Create one and sel ect all folders, although you may not want the trash folders if you want deletes to disappear.
Also if you reply or forward fr om a VF, the original account settings are used