Sass has two syntaxes. The new main syntax i.e. Sass 3 is known as SCSS (Sassy CSS) and is a superset of CSS3 syntax. This means that every valid CSS3 stylesheet is valid SCSS as well. SCSS files use the extension .scss.
The second and older syntax is known as the indented syntax i.e Sass. This is inspired by Haml’s terseness and is intended for people who prefer conciseness over similarity to CSS.