A style, such as this Heading 1, consists of many formatting details.
You might be used to applying all your formatting in many small steps. For example, to format a heading, you could use the Bold button and the Font and Font Size boxes on the Ribbon’s Home tab. Or you could open the Font dialog box and select more options there, such as an underline style.
You could do it this way, but it's a lot of keystrokes. And what about when you want to use the same formats for another heading?
This is where styles come in. They're specially crafted packages of formatting that apply many attributes at once. Instead of applying each thing individually, you apply the style once and you're done.