Sass is a CSS pre-processor with syntax advancements. Style sheets in the advanced syntax are processed by the program, and turned into regular CSS style sheets. However, they do not extend the CSS standard itself.
CSS variables are supported and can be utilized but not as well as pre-processor variables.
For the difference between SCSS and Sass, this text on the Sass documentation page should answer the question:
There are two syntaxes available for Sass. The first, known as SCSS (Sassy CSS) and used throughout this reference, is an extension of the syntax of CSS. This means that every valid CSS stylesheet is a valid SCSS file with the same meaning. This syntax is enhanced with the Sass features described below. Files using this syntax have the .scss extension.
The second and older syntax , known as the indented syntax (or sometimes just “Sass”), provides a more concise way of writing CSS. It uses indentation rather than brackets to indicate nesting of selectors, and newlines rather than semicolons to separate properties. Files using this syntax have the .sass extension.
However, all this works only with the Sass pre-compiler which in the end creates CSS. It is not an extension to the CSS standard itself.