A style sheet defines design and layout information for a document. Commonly, style sheets also specify fonts, colors, indentation, kerning, leading, margins, and even page dimensions for any associated documents that employ a particular style sheet.
In the publishing world, style sheets are indispensable. Style sheets enable numerous people to work on large projects. All participants can work at their own systems, independently of other project team members. When the pieces of the project are brought together, the final product derives its consistent look and feel (or design and layout, if you prefer typographically correct language) because a common template (what style sheets are often called in the print industry) ensure a common definition for the overall document.
This consistency is a highly desirable characteristic of final products for both print and electronic (online) documents. The World Wide Web Consortium (W3C) has therefore made a gallant effort to incorporate the consistency of print on the Web by introducing style sheets. With the use of Web style sheets, an entire Web site can look the same on every platform and within every browser that supports style sheets. For Web authors who want to provide a consistent reading experience, this promises to be a vast improvement over the current, somewhat more chaotic state of affairs.