CSS - Cascading Style Sheets
CSS is a shortcut for "Cascading Style Sheet" and it is used to define styles that determine the look of HTML elements (font, colours, backgrounds, space...).
Those styles are added to "Style Sheets", extern files with .css extension or simply write in the heather of HTML documents, or even inline, directly on the elements.
Extern styles are the best because they save your working time, and redesigning HTML document time.
CSS was originally created in W3C, and it showed up together with HTML 4.0 as a solution for a problem in separation of page content from design.
HTML was imagined to take care just for the content - it was supposed to show only paragraphs, tables, headings, etc. The appearance was covered by browsers.
But, many browser creators started adding additional HTML tags (such as font tag and colour attribute) to original HTML, many developers found themselves difficult to create their own pages.