CSS stands for Cascading Style Sheets and is a style sheet language that is used to design and format HTML documents. With CSS, web designers can control the appearance of elements on a website, such as colours, fonts, spacing, sizes and layouts.

The main advantage of CSS is that it allows designers to centrally control the appearance of an entire website by storing formatting information in one or more separate CSS files. This separation of content (HTML) and design (CSS) facilitates maintenance, improves loading times and enables a consistent design across multiple pages.