Style sheets are a centralized method of regulating the appearance or use of design elements and/or wording. Style sheets take several different forms:

1. In layout or word-processing software such as Adobe InDesign, QuarkXPress or Microsoft Word, style sheets are used to create predefined sets of formatting instructions (and, rarely, additional content). For example, a style called "headline" can be created that specifies how a user wants all the headlines to appear. The style can include which font the headline appears in, how large, how much leading it should have, what color it is, how much paragraph spacing or letter spacing, etc. Any text tagged with that style will take on that formatting. Later, if the user wants to change one or more aspects of the headlines in the document, such as what color they are, the user needs only to change the definition of "headline" in the style sheet, and all of the tagged text in the document change accordingly. Styles in layout programs typically apply to units of paragraphs, words, characters, or image elements.
2. In web design, style sheets have the formal name of Cascading Style Sheets (CSS). CSS works much the same way as style sheets for layout programs and provides greater typographic control than what is built into unstyled HTML, with control over font selection (with generic fallback specifications like sans-serif), size, leading, paragraph spacing, color, and letter spacing, among other features. By establishing, for example, a headline style in CSS (applying to any of the h1 through h6 HTML elements), all of the user's headlines on a site can be consistent and easily changeable through a single entry in the CSS file. Since CSS separates presentation instructions from the structural HTML of the page, sites that use CSS are often much leaner. A good site that shows what can be done with CSS is CSS Zen Garden.
3. Style sheets are also used as internal guidelines within an organization to maintain a consistent writing style. These are more usually referred to as style guides, style manuals or style books. These kinds of style sheets regulate whether an organization uses the serial comma (the final comma in a series, either "this, that, and the other thing" or "this, that and the other thing"), what kinds of works get italics or quotation marks for their titles, whether and/or how small caps are used for abbreviations, etc. Two of the major style guides are the AP Stylebook and the Chicago Manual of Style.