CSS stands for Cascading Style Sheets.Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation semantics (the look and formatting) of a document written in a markup language.Cascading Style Sheets (CSS) is a simple mechanism for adding style (e.g., fonts, colors, spacing etc.) to Web documents.Before you continue you should have a basic understanding of the following: HTML/XHTML
HTML was intended to planned the content of a document, like: <h1>This is a heading</h1> <p>This is a paragraph.</p>
If we apply formatting like backcolor, bold, italic etc. we have to apply every page and every html tag. so this is long process.To solve this problem, the World Wide Web Consortium (CM) created CSS.A css format can be applied for every page and every tag. In HTML 4.0, all formatting could be removed from the HTML document, and stored in a separate CSS file. All browsers support CSS today.