Sass is a CSS preprocessor that adds powerful features like variables, nesting, mixins and functions to regular CSS. It makes CSS more maintainable and scalable. There are two syntax options for Sass - SCSS which uses the familiar CSS syntax, and the indented Sass syntax which relies on indentation rather than brackets. Sass code can be compiled into regular CSS files using the sass command line tool. Key Sass features include variables, nesting rules, partials and imports, mixins, extends/placeholders, interpolation and control directives.