It is essentially the clothing on a webpage. It defines the appearance of a web page. It is simply a text file that contains one or more rules that determine through properties and values how certain elements in your web page should be displayed.
The language is called Cascading Style Sheets because styles are inherited from tiers above. They are passed down from the three places to write CSS: in an external style sheet, embedded in the head of an HTML document, and inline.
It also makes it much easier for updating websites. It can be created outside of a web page and then applied to all the pages on your site at once. It is flexible, powerful, and efficient and can save you lots of time and bandwidth.
To get the full benefit of CSS, your web pages must be well structured in HTML first.
Check out csszengarden.com and/or right-click on a web browser and Inspect the page to see the full power of CSS.