CSS (Cascading Style Sheets) is a powerful web design language used to control the appearance and layout of HTML documents. It allows web developers to define styles such as colors, fonts, background images, margins, borders, and positioning of elements on a webpage. CSS separates the design from the content, making websites easier to maintain and faster to load. There are three main ways to apply CSS — inline CSS, which is written inside an HTML element; internal CSS, which is written within the <style> tag in the HTML document; and external CSS, which is linked through a .css file.

Using CSS, developers can create responsive designs that automatically adjust to different screen sizes, providing a better user experience across devices. Advanced features like flexbox, grid layout, and animations make it easier to design complex and dynamic web pages. Overall, CSS is an essential part of modern web development, giving websites a professional look and consistent styling.