Cascading Style Sheets (CSS) make it faster and easier to create and maintain websites. CSS works with HTML (or any markup language) to apply a uniform style and format to your website.
You can embed CSS into a single Web page, or you can link your Website to an external CSS file. Whenever you update the file, every page that links to it also updates. Therefore, you can set your Website's fonts, colors, layouts and more without repeating the HTML formatting tags throughout.
To learn more about CSS, go to W3Schools.com.