Both Sass and LESS are CSS preprocessors, which allow writing clean CSS in a programming construct instead of static rules.

LESS is inspired by Sass. Sass was designed to both simplify and extend CSS, so things like curly braces were removed from the syntax. LESS was designed to be as close to CSS as possible, so the syntax is identical to existing CSS code. As a result, existing CSS can be used as valid LESS code.

The newer versions of Sass also introduced a CSS-like syntax called SCSS (Sassy CSS).