A style sheet language used to define the layout and appearance of web pages. The purpose of CSS is to allow for the separation of document content (HTML) from it’s appearance (CSS) allowing for the easy repurposing of the content for different devices and media. It also allows for multiple documents to share a style sheet improving efficiency for both the developer and the end user.
Cascading refers to the manner used to determine the priority of style rules when more than one rule applies to a given page element.
The term “style sheet” is a bit misleading since CSS rules are not required to be grouped together in one file and may, in fact, be used within an HTML document (although best use generally calls for styles to be grouped into one or more external CSS files linked to the HTML file).
The CSS standard is maintained by the World Wide Web Consortium (W3C). The first widely adopted version of CSS was CSS2 which was published as a W3C Recommendation (the equivalent of a standard) in 1998.
CSS3 is actually a series of modules rather than a single specification. Several CSS3 modules, most notably media queries, have been released as W3C Recommendations. Support of CSS3 varies widely among browsers.