"What is it? and what does it do?"
CSS stands for Cascading Style Sheets. It is a style sheet language used for describing the presentation of a webpage that has been written in a markup language. This includes colors, layouts, and fonts. It allows the developer to adapt the presentation to different types of devices such as desktop, tablet, and phone screen sizes. CSS is independent of HTML and can be used with any XML-based markup language.
CSS3 is the latest evolution of the CSS language, extending CSS2.1 and completely backwards compatible with previous versions of CSS. It includes long awaited features such as rounded corners, shadows, gradients, transitions, and animations; plus new layouts like multi-columns and flexible grid layouts.