CSS
CSS - Cascading Style Sheets
CSS
CSS is a style sheet language used for describing the presentation semantics (the look and formatting) of a document written in a markup language.

Prior to CSS, nearly all of the presentational attributes of HTML documents were contained within the HTML markup; all font colors, background styles, element alignments, borders and sizes had to be explicitly described, often repeatedly, within the HTML. CSS allows authors to move much of that information to another file, the style sheet, resulting in considerably simpler HTML.
Reference
Wikipedia
Common Web Languages
CSS
CSS3
HTML
HTML5