Sass (syntactically awesome style sheets) is a scripting language that is an extension of CSS.

It enriches CSS with additional features and makes larger and more complex stylesheets easier to maintain.

It is available in two syntax variants. The .scss format, which like .css, uses parentheses and semicolons. The second, more readable form is .sass, which uses indentation (similar to HAML or Python).

Both syntaxes are finally compiled to the .css format.

More info:

Wikipedia: https://en.wikipedia.org/wiki/Sass_(stylesheet_language)