There may be instances, particularly with interactive Widgets, that you want to convey a different look for your Widget based on how it is interacted with or the condition it is in. For example, say you have a button on screen that appears normally and when moused over, changes colors or pulsates, then when clicked does something entirely different. Referred to as States, this is the most common form of styling and allows you to specify how the Widget appears based on the current state it is in.
Above, a Button Widget changes based on its Normal, Pressed, Hovered, or Disabled States. Most Widgets will use the Normal, Pressed, Hovered, and Disabled States, however there may be additional options based on the type of Widget you are using.