CSS (Cascading Style Sheets) is a stylesheet language used to control the appearance of HTML elements on a webpage. It allows you to add colors, fonts, spacing, layout, and responsiveness to your site. By linking a CSS file or using inline styles, you can separate content from design, making websites easier to maintain. Key concepts include selectors, properties, and values, which let you target elements and style them. Learning CSS is essential for web development, helping you create visually appealing and user-friendly websites that work well on all devices.