CSS, standing for Cascading Style Sheets, is a simple way to organize the styling attribute options for the HTML elements used on a page. Typically stored in a .css file and loaded at the <head></head> part of the web page, it applies all definitions and options to the whole document in just the same way you would decorate your flat. Styling can be applied either on DOM elements directly, on classes, on IDs, or on event triggers such as clicking or hovering.
Looking at the official logo of CSS, it closely resembles that of HTML5, which indicates they are meant to work in conjunction. The latest version of CSS is CSS3, but the version number, in this case, is immaterial as it's a simple extension of its older versions with some additional attributes which are compatible with HTML5.