Design System (Component Library)
A design system is a documented collection of reusable interface building blocks such as buttons, form fields, cards and navigation, together with the rules for when to use them. It links the design source with the actually coded component, so that mock-up and live site do not drift apart.
In practice
The benefit is commercial: new pages and campaigns are assembled from existing blocks instead of paying for design and development again each time. For small sites the effort does not pay off, but from the point where several people create content, or several locations, languages and campaign pages appear, it pays back quickly. The most common mistake is building a system once and never maintaining it, after which three variants of the same button exist within a year. Make sure accessibility is built into the components, so that it does not have to be worked out again on every page. Testing the finished page still remains necessary: alternative texts, heading structure, reading order and the contrast of editorial content are decided outside the components. When reviewing a quote, ask explicitly whether a design system is included or pages are designed individually.