The Categories module enables users to sort their documents based on categories. It is thus a different approach to sorting documents from the one where documents are sorted using tags.
There are three types of categories from the point of view of the user:
•Global categories - such categories can be used across all available sites.
•Site-specific categories - such categories can be used only on the given site.
•Personal categories - global categories belonging to a particular user. The user can use these categories across all available sites.