Custom form locations, and what exactly is the 'Outlook Folders' entry in the list of custom form locations?

In Outlook 2010, when I look for a custom form, there are several locations to look in, for example

Standard Forms Library
Organizational Forms Library
Personal Forms Library
Outlook Folders

What exactly are the Personal Forms Library and the Outlook Folders representing?

There is a custom form that is used in the company, but I don't see it in the Organizational Forms Library. From the various outlook clients, the form is available for use in "Outlook Folders". I need to make sure that everyone is using the same version of the form.