A predefined format that you can apply by clicking a button in the style group on the home tab. You can click on the MORE drop down arrow.
Explanation:
A style is a predefined combination of font style, color, and size that can be applied to any text in your document. Styles can help your documents achieve a more professional look and feel.
Style sets include a combination of title, heading, and paragraph styles. Style sets allow you to format all elements in your document at once instead of modifying each element separately.
The more drop down arrow shows a variety of styles which you can use in your document.
It allows you to Navigate through the pane and also add a table of contents.
You can check the design tab for extra options, if you don't like any of the contents in the More drop down box.
After selecting the desired style from the drop down, the text appears in the style selected by you.