A bar category defines a bar style that can be used with the Bar Chart. The categories can be grouped using bar groups , in order to create effects like the following :
As you can see , each category defines a bar style. In this case the categories are the blue the green and the yellow bar series. The categories are then grouped so you can have more then 1 value for each.
The following articles cover the settings and use of bar categories and groups in Graph and Chart.