CSS, or Cascading Style Sheets, is the language used to control the appearance and layout of web pages. While HTML defines the structure of content, CSS determines how that content looks. It allows developers to adjust colors, fonts, spacing, borders, backgrounds, and element positioning.

CSS works by using selectors to target HTML elements and apply styling rules. Developers can style individual elements, reusable classes, or entire sections of a website. The cascade determines which declarations take priority when multiple rules apply, helping browsers resolve overlapping styles.

Responsive design is an essential part of CSS. Media queries allow styles to adapt to different screen sizes, while flexible layouts help content fit the available space. Tools such as Flexbox and CSS Grid make it easier to organize navigation menus, cards, galleries, and page layouts.

CSS also supports transitions, animations, and interactive states such as hover and focus. These features can provide visual feedback and make interfaces easier to use. By combining consistent styling with readable typography and adaptable layouts, developers can create attractive, accessible websites that work across devices.