Event is a monitoring of an interaction within some certain parts of the website.
A perfect example of an event tracking would be a SignUp button or a LiveChat Button.
Events are user interactions with content that can be tracked independently from a web page or a screen load. Downloads, mobile ad clicks, landline tracker,gadgets, Flash elements, AJAX embedded elements, and video plays are all examples of actions you might want to track as Events. Category
A category is a name that you supply as a way to group objects that you want to track. Typically, you will use the same category name multiple times over related UI elements that you want to group under a given category.
Suppose you also want to track how many times the video is downloaded. You could use: