CSS stands for Cascading Style Sheets and is used to define the presentation and layout of web pages. CSS allows separation of style from content, making pages easier to maintain as they grow. There are three main ways to add CSS to HTML - inline, internal/embedded, and external. Inline CSS is applied directly to HTML elements using the style attribute, internal CSS defines rules within the <style> element in the HTML head, and external CSS links to a separate .css file.