CSS stands for Cascading Style Sheets. CSS was first introduced in 1996. Throughout the past years, a lot of changes have been made to the CSS and as of current, we are using CSS3 which is the latest version. CSS3 is also known as CSS level 3.
The structure of a web page depends upon HTML, but its look and feel depends upon CSS. However, we do have an option to use attributes in HTML for enhancing the appearance of a web page, but CSS3 can do a lot more than those attributes can do.
W3C (World Wide Web Consortium) maintains CSS specifications. These specifications are further followed by web developers and the organizations who make browsers like Google Chrome, Safari, Mozilla Firefox etc, so that web page should render properly to the end user.
With CSS and HTML, you can easily separate Styling and Structure of web page. This means a CSS file will take care of the styling and a HTML file will take care of the structure.