CSS stands for *Cascading Style Sheets*. It is a language used for describing the style of a document written in HTML. It was designed to enable the separation of presentation and content, which greatly improved content accessibility and gave more flexibility and control in the specification of presentation, and reduced complexity and repetition in the structural content. It was first invented in 1996, and is now a standard feature of all major web browsers. The newest version of CSS is CSS3, which builds upon CSS2.1 and adds more visual functionalities, ready for the modern world.
CSS specifications are maintained by the [World Wide Web Consortium (W3C)](https://www.w3.org/).
You can build some pretty amazing things in CSS alone, such as this pure-CSS [Minesweeper game](https://codepen.io/bali_balo/pen/BLJONk) (which uses no JavaScript).
[Jon Duckett's book on HTML and CSS](https://www.amazon.com/HTML-CSS-Design-Build-Websites/dp/1118008189) could be an exellent start for the ones who wish to explore the topic to it's great depth with some amazing examples.
Another suggestion for beginners is W3C's [Starting with HTML + CSS](https://www.w3.org/Style/Examples/011/firstcss) teaches how to create a style sheet.
The site [CSS Zen Garden](http://www.csszengarden.com/) is a great example of how the same HTML code/structure can be styled to look different in unique ways.
When you're practice and working with CSS, you can refer to this [CSS reference](https://cssreference.io/) to help you along the way if you're unsure about a property. A great tool for helping visualize what each CSS property does.
