CSS stands for Cascading Style Sheets.

CCS The Casacading Style Sheets Specipication (css) is a computer language that is used to write formatting instructions that is used to write formatting instructions (rule). These rules tell a web browser how webpage content should 'look' - in terms of :

position, alignment, width
height, etc.
typeface, font-weight,
color, border, etc.

Stylesheet CSS rules are added to a webpage either by writing the code directly into the of the webpage HTML, or by linking to separte file.
A sperate file containing only CSS rule is commonly referred to as a 'stylesheet' and has the extension css(dot-C-S-S)

CSS is the acronym for "Cascading style sheet". CSS is a simple design language intended to simplify the process of making web pages presentable.
CSS handles the look and feel part of a web page. using CSS, you can control the color of the text, the style of fonts, the spacing between paragraphs, how columns are sized and laid out, what background images or color are used, layout design, and variations in display for different devices and screen sizes as well as a variety of other effects.

This tutorial covers give a complete understanding of CSS, starting from its basic to advanced concepts

CSS Modules