CSS3 have many options and features, but it has a low degree of re-usability. SASS is designed as a language designed to ease and improve CSS3 writing.
SASS is a Ruby program that given SASS files it builds for you a CSS3 files that can be used directly in any web or app. The strong points is that it allows to use functions and variables. It also has connectors to integrate it directly with Ruby servers or other technologies like NodeJS or GruntJS.
SASS language is similar to Ruby language which makes code cleaner and easier to understand. In addition it also defines the SCSS which is exactly like CSS3 but it allows to use all SASS features. It makes easier the migration from CSS3 to SASS/SCSS.