CSS, or Cascading Style Sheets, is a style sheet language used for describing the presentation of a HTML document . It is a fundamental technology used on the web to control the layout, design, and visual appearance of websites.

It allows web designers and developers to separate the content of a website from its presentation, making it easier to manage and update the appearance of a site. By using CSS, the same HTML document can be presented in different styles for different devices. These devices can be desktops, mobile phones, and tablets.

It works by applying style rules to elements in the HTML document. These rules specify how the elements should be displayed, such as their color, size, font, spacing, and positioning. CSS can also be used to create animations, transitions, and other visual effects on a webpage.

One of the key features of CSS is its ability to cascade styles, meaning that different style rules can be applied to the same element. This allows for the creation of complex and customized designs while maintaining a consistent and organized style sheet.

In summary, CSS is a crucial aspect of web design and development, allowing for the creation of visually appealing and responsive websites. Understanding how to use CSS effectively can greatly enhance the presentation and usability of a website.