SASS is a scripting language that can be compiled into CSS. Biggest competitor of a similar language LESS. SASS is more alike programming language, and as such has a few advantages: logical and looping operators, simplified @media blocks, strong mathematical functions. Almost essential for SASS is the Compass library, which implements a lot of common web development design tasks.
However, these are also the reasons why SASS is less popular, as it is harder to learn and get used to, and being a compiled language it is not as straightforward to convert to CSS as LESS. SASS code is also harder to read and maintain, and without proper discipline can turn into a nightmare to support.