CSS (Cascading Style Sheets) is used to style and lay out HTML elements. CSS can control the presentation of multiple web pages at once. There are three ways to add CSS - inline using the style attribute, internally using the <style> element, and externally using a separate CSS file. The most common method is to define styles in an external CSS file and link it to HTML pages. Inline CSS is used to style a single element, internal CSS styles an individual page, and external CSS styles entire websites. CSS can style elements with properties for borders, padding, margins, and IDs or classes.