CSS stands for Cascading Stylesheet. Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing) to HTML documents or web pages. CSS handles the look and feel part of a web page. Using CSS, you can control the color of the text, the style of fonts, the spacing between paragraphs, how columns are sized and laid out, what background images or colors are used, layout designs,variations in display for different devices and screen sizes as well as a variety of other effects.
CSS3 extends CSS2.2 (or rather CSS 2) and it is the latest upgrade to the Cascading Style Sheets language. It brings a lot of new features and additions. Some of these features include rounded corners on HTML elements, shadows on divs and texts, gradient-fills, animations, as well as new layouts like multi-columns, flexible box or grid layouts.