The category will be the name that has been assigned to the group of events that the web site owner wants to track. This could be any category that is specific to the business including YouTube videos, podcasts, profitable engagement or reading. The total amount of events logged in each category could be further broken down into subcategories if it was conducive to the business needs.
Categories of events in Google Analytics include mouse, keyboard, frame and form events. Two broad categories exist over this, one which generates pageviews when they occur and one which does not generate page views when they occur.
An example of this would be a clothing retailer who categorized events based on purchases of a certain piece of clothing. They might want to break the categories further down from simply female or male clothing to shirts, jeans, shoes etc.