Sass makes CSS fun again. Sass is an extension of CSS3, adding nested rules, variables,mixins, selector inheritance, and more. It’s translated to well-formatted, standard CSS using the command line tool or a web-framework plugin.
Sass has two syntaxes. The new main syntax (as of Sass 3) is known as “SCSS” (for “Sassy CSS”), and is a superset of CSS3’s syntax. This means that every valid CSS3 stylesheet is valid SCSS as well. SCSS files use the extension .scss.
The second, older syntax is known as the indented syntax (or just “Sass”). Inspired by Haml’s terseness, it’s intended for people who prefer conciseness over similarity to CSS. Instead of brackets and semicolons, it uses the indentation of lines to specify blocks. Although no longer the primary syntax, the indented syntax will continue to be supported. Files in the indented syntax use the extension .sass.
It has everything I ever wanted CSS to have. It helps you stays DRY. Moreover it has Compass which helps you make standard things in your UI (like rounded corners) staying browser-agnostic.