Sass is an extension of CSS3. It is translated to well-formatted, standard CSS using the command line tool or a web-framework plugin. Most commonly used syntax of sass is "scss". SCSS files use the extension ".scss". The other syntax is "sass". We will talk about scss here.
Advantages of scss over css
Start with learning object-oriented PHP that you need to understand Drupal 8 code. The free eBook covers:
Can highly recommend the @redcrackle Drupal 8 #OOP intro found at http://redcrackle.com/blog . Also available as handy PDF. Very well written!