Categories associate content with keywords for easier searching.
Categories are words or phrases predefined by administrators within a particular place (a space or social group). If there are categories available in the place, it usually means that the place's administrator has given thought to how that place's content should be grouped. An administrator who creates a category can optionally associate certain tags with it behind the scenes.
How do I apply categories?
You can assign categories to your existing or new content. To assign your content to categories:
Open the content for editing and scroll to the bottom.
Beneath the editing window, under Categories, you might see a list of categories that have been defined for the place in which the content lives. Select check boxes for the categories in which your content seems to belong.