As Defined by W3 Schools…

What is CSS?
CSS stands for Cascading Style Sheets
CSS describes how HTML elements are to be displayed on screen, paper, or in other media
CSS saves a lot of work. It can control the layout of multiple web pages all at once
External stylesheets are stored in CSS files

As Defined by Rock Intuit…

The best way to develop a website is with a responsive layout. CSS 3 makes website changes, page animation, further development and expansion simple and completely user friendly! In short, say goodbye static pages and hello responsive design!