A design system is a living library of interface components together with rules for their use – a shared source of truth from which designers and developers build consistent digital products faster and without guesswork.
A design system is more than "a nice kit in Figma". A complete system includes: design tokens (values stored as data), components with variants and states (in code too!), patterns (how to build forms, how to communicate errors), documentation and a governance process.
Why would a company want one? First, consistency – the product looks and behaves the same in every module. Second, speed – teams assemble new screens from ready blocks instead of designing from scratch (30–50% time savings are typical). Third, quality – components solve accessibility and responsiveness once, properly.
The best design systems are alive: they have an owner, versioning and a change process – like any good product.