CSS

CSS stands for Cascading Style Sheets and it is really what separates the professionals from the rest of the pack of web designers. Wikipedia defines CSS as:

"Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in a markup language. Its most common application is to style web pages written in HTML and XHTML, but the language can be applied to any kind of XML document, including SVG and XUL. The CSS specifications are maintained by the World Wide Web Consortium (W3C)."

In essence it means that the presentation of web page is separated from the content of a web page (or any of the above mentioned formats).

In the early days of the Web, a page would consist of the content of that page and all the formatting of that page.