Style sheets are collections of rules for modifying how a SGML document appears. Cascading Style Sheets (CSS) are the specific implementation adopted by the W3C for HTML.
The core concept of CSS is that defines rules altering the appearance of HTML elements that can be selectively applied. These rules are held in a document (the style sheet) and are applied in a well-defined priority order (the cascading part of CSS).
As of CSS Version 3, CSS technologies were split into separate modules allowing them to be revised and maintained separately. Each module adds or extends features to those defined in CSS Version 2, in order to maintain backwards compatibility.