At the risk of oversimplification, Compass is a framework while Sass is a language abstraction. You write Sass code (and compile it to standard CSS) within the Compass context. SCSS is simply the most recent syntax of the Sass language. It was created for the sole purpose of being more CSS-like in its syntax, as far as I can tell. All the syntactic sugar of Sass, but more familiar to front end developers comfortable with CSS.