What's the difference between a design system / style guide and pattern library?
These terms are related but distinct:
A style guide defines your visual language—like brand colors, typography, and usage rules. It's usually static and focused on aesthetics.
A pattern library (or component library) is a collection of reusable UI elements, like nav bars, buttons, or forms, designed for consistency.
A design system is the complete framework. It includes the style guide and pattern library, plus coded components, usage documentation, accessibility guidelines, and more.
In short, a design system unifies design and development—it's the full toolkit for building consistent, scalable products.