What CSS is
CSS or Cascading Style Sheets is a web technology and web language that is used to style web pages in the form of HTML files.
CSS is a very powerful markup language, meaning that it is not a programming language as such, but something that gives directives to a client's web browser on what to do.
CSS is an easy to learn language - it's specifications make it that way.
It was designed as an easy alternative to writing the same styles over and over again in HTML by two computer scientists - Håkon Wium Lie and Bert Bos.