CSS stands for cascading style sheets, a style language used to add fonts, spacing, color and other stylistic elements to documents on the Internet. CSS is used to style documents written in markup languages such as HTML, XHTML and XML.
In a typical application of the technology, a CSS document exists as a text file that controls the styling of many associated content documents. A change in the CSS document produces a change in all associated content documents. With CSS in place, a Web developer can change a stylistic element such as a text font in hundreds of documents instantaneously through a single change in the CSS document.