SASS is a CSS pre-processor that allows the creation of stylesheets in a faster and more readable way. It has two types of syntax: original for SASS and extension for .scss. The main benefits of using the pre-processor are:

• multi-level nesting (one css rule can contain another),
• variables,
• calculations,
• mixins (fragments of css code that can be used repeatedly),
• importing styles and organising many css files,
• the ability to write your own functions or loops, which allows you to quickly create universal css rules.

An alternative to Sass is Less or Stylus, but these are much less frequently used.