SASS and SCSS are both CSS preprocessors that add features like variables, nesting, and mixins to regular CSS. SASS uses indentation for its syntax while SCSS uses brackets and semi-colons like regular CSS. SCSS was introduced as the new main syntax for Sass and is a superset of CSS, so any valid CSS is also valid SCSS. Both SASS and SCSS have the same features but SCSS has a more familiar syntax for CSS authors.