Sass is a preprocessor and extension language for CSS that compiles into standard stylesheets browsers understand. It adds variables, nesting, mixins, inheritance, functions and a module system, so repeated colors, spacing scales, breakpoints and component patterns can be defined once and reused across an entire design system.
The official site documents both the indented syntax and the SCSS syntax, the modern @use and @forward module rules, built-in modules for color, math, string and list operations, the command-line interface, and integration guidance for build tools such as Vite, webpack and common framework toolchains.
It suits front-end developers maintaining large or multi-brand style sheets, design system teams encoding tokens and component variants, and agencies that need to share a consistent styling foundation across several projects without copying rules between files.
Compared with writing plain modern CSS it adds logic and reuse but requires a compile step; compared with utility-first CSS frameworks it controls the underlying rules rather than prescribing class names; many native CSS features such as custom properties now overlap with variables, so the two are often combined.
Prefer the module system over the deprecated @import rule, keep mixins focused to avoid generated CSS bloat, and pin the compiler version in the project because deprecation warnings between major releases can change how certain functions and color operations behave.