SASS, or Syntactically Awesome Style Sheets, is a CSS Pre-Processor, which compiles into CSS for web browsers to understand.
It was built to allow developers to create code blocks for repeating styles, using ‘mixins’. These mixins can be called within style elements to reduce having to duplicate styles for different class names.
A newer syntax of SASS, called SCSS, uses formatting very similar to CSS, but with the power of variables, nesting, and extends. Early versions of SASS didn’t compile into correctly formatted CSS, however, recent versions allowed developers to define how the compiled CSS is displayed. SCSS is also fully compliant with the CSS syntax specification, whereas SASS is not compliant with the CSS syntax.
I use the SCSS format, utilising variables, nesting and extends, focussing on clean code in the outputted CSS.