Style Sheets, or Cascading Style Sheets, CSS, are used to add styles to the HTML which used to define the content of web pages. The style sheets add style and layout control to the default user interface provided by the browser.
You can start learning about style sheets here. This is a tutorial for beginners to HTML coding.
Style Sheets, in its most recent version, CSS3, is part of the HTML5 web technology which also includes the latest version of HTML and the proramming language, JavaScript.
You can start learning about HTML here and JavaScript here. Again they are both tutorials for beginners.