A cascading style sheet (CSS) is a Web page derived from multiple sources with a defined order of precedence where the definitions of any style element conflict.
CSS is designed primarily to enable the separation of document content from document presentation, including aspects such as the layout, colors, and fonts. This separation can improve content accessibility, provide more flexibility and control in the specification of presentation characteristics, enable multiple HTML pages to share formatting by specifying the relevant CSS in a separate .css file, and reduce complexity and repetition in the structural content. The style definitions are normally saved in external .css files.
With an external stylesheet file, you can change the look of an entire website by changing just one file.
You can think of HTML as the bones on a website and css as the skin and makeup to the website.