As a beginner you may style only the hover states and forget about active and focus.
focus is important in form design, this comes into play when you are tabbing through the input items and focus tells you what field is selected.active is important in buttons, this is when you have clicked on a button and haven’t let go of the mouse yet (the click is not yet over). active tells you what you are actively pressing on..hover, active and focus can be grouped together and hover and focus are usually the same.