What are Sass, LESS, and Stylus? Why do people use them? How does something like Compass relate to Sass?

Sass, Less and Stylus are css preprocessors, like browser processes our css and html to generate webpage, preprocessors process the code to produce css before its handed over to browser and then browser does its work.

The Compass is an extension to SASS, which adds additional functionality to Sass e.g. Compass has a bunch of mixins which we can use and reduce our time of writing code.