A system is a set of refusals

A component library is a folder of components. A design system is that plus the rules that decide what a new component may look like, which sizes exist, which spacings exist, what a surface is, what an accent means. The second is what stops the fortieth component from being subtly different from the first.

The rules are almost entirely refusals. No size outside the scale. No colour outside the tokens. No radius outside the three we chose. Any new pattern gets added to the system before it gets used a second time. Enforced properly, this makes consistency the default rather than a review comment.

Not everything belongs in a system. One-off marketing pages, experiments, and anything you have built exactly once should stay outside it. Promoting something to the system is a commitment to maintain it, and a system full of single-use components is worse than no system, because now everyone has to read it.