Inspired by Brad Frost's Atomic Design, many organizations today have started to adopt design systems - a single source of truth which group all the elements that will allow teams to design, realize and develop a product.
This not only includes the stylesheet and the components but also the voice and tone to use, the overall principles, use cases for each component, and the dev framework (this may vary from team to team).
Rather than writing more, we recommend that you read the following three articles to get a solid understanding of the subject as many UX Designers are expected to work with design systems @ work.
A perfect start to understanding the subject better, this article covers the basics of design systems and the need for one.
A natural next read, this article uses strong visuals to give you a step by step detailed overview of the journey of a design system from start to finish.
Rather than getting more textbook definitions and explanations, this dribble blog post takes a different approach and shows you how design systems vary from organization to organization and how there is no one correct/perfect way of making a design systems.
The Design Better team has collaborated with UX designers from Etsy, Github, and inVision to deliver this perfect book to accompany you in creating your design system.
A great way to start designing your own design system is to explore pre-existing design systems. Doing this will help you identify what types of components, rules, and pattern guidelines are usually included in a design system. Our favorite design systems to explore for guidance are Material Design and EOS. Here is a list of the most popular design systems out there.