CSS – Cascading Style Sheets
CSS is a computer language that describes to a browser how a web page should be displayed.
HTML is used to describe the structure of a page but not how it is displayed. An HTML web page can be displayed without using any CSS and the web browser will use its default styles for the HTML tags used. This will result in a very minimal and uncolourful display but the document will still be well set out and readable.
To make the web page more interesting to look at CSS is used to define how each HTML tag will be displayed. There are a multitude of different settings that can be used, including: foreground colour, background colour, size, borders, padding, margins, alignment and background images.
Well written and standards compliant web pages will have the CSS in a different file to the HTML. The main HTML page will link to a CSS file. The same CSS can then be used for every HTML file. Even though the contents of the HTML files will be different the CSS will ensure they all use the same layout and style.