SCSS (Sassy CSS) is a CSS preprocessor that is an extension of the syntax of CSS. It adds features such as variables, nesting, and mixins, which make it easier to write and maintain large CSS codebases. SCSS is fully compatible with CSS and can be used alongside it.
Sass (Syntactically Awesome Style Sheets) is the original CSS preprocessor and it was first release in 2006. It provides the same features as SCSS, but uses a different syntax. Sass uses indentation instead of curly braces to indicate nesting and uses colons and semicolons differently.
Both SCSS and Sass provide the same functionality, but have different syntax, you can use either depending on your preference.