CSS stands for Cascading Style Sheets and is a language used to describe the presentation of a document written in HTML or XML. It is used to style and layout web pages, including altering the font, color, size, and spacing of content, splitting it into multiple columns, or adding animations and other decorative features. CSS is unique in that it doesnt create any new elements, like HTML or JavaScript, but instead is a language used to style HTML elements.

CSS allows for the separation of content and presentation, making the styles their own file rather than mixing in with the HTML. This means that the same HTML document can be styled in many different ways and can remain accessible to different rendering methods, such as screen readers. CSS is responsible for the text style, size, positioning, color, and more on a website, and it controls how a websites style shifts between desktop and mobile versions.

Advantages of using CSS include saving time, as once a style sheet is created, it can be used multiple times, and the best practice for CSS is to save it as a .css file, separate from the .html file. CSS also allows for the creation of a layout, such as turning a single column of text into a layout with a main content area and a sidebar for related information.

In summary, CSS is a language used to style and layout web pages, allowing for the separation of content and presentation and providing advantages such as saving time and creating layouts.