CSS is used to style a website, declaring colours, fonts, and layout of the page to the web browser. It was originally released in December 1996, to separate content from style. This allowed websites to become more accessible, as a lot of data for the styling of a website was removed from the HTML.
Separating style from content allowed pages to load faster, as the main content was being loaded while the styling was loading at the same time. This was an important step as it allowed users to turn off styling on slow connections, and get quicker access to to what they wanted, the content.
CSS is now in its 3rd version of the specification, with the 4th currently in development by the World Wide Web Consortium (W3C). CSS3 is still being added to Web Browsers, although the standard has been finalised. The specification for CSS3 started to be released in 2011, with Media Queries added in 2012.
I use CSS3 to style websites, making use of the features in the latest release.