System design is the process of deciding how software components cooperate to deliver a product under real constraints. It covers responsibilities, interfaces, data ownership, deployment, failure handling, security, and operational behavior—not just drawing boxes or choosing a database.

Imagine building a URL shortener. The visible feature is simple: accept a long URL and return a short code. A production design must also answer questions such as how codes are generated, how redirects stay fast, how abuse is prevented, what happens when a database is unavailable, and how the service scales during a traffic spike.

A system is a collection of interacting parts. A design is the set of decisions and trade-offs governing those parts. Good design is contextual: a small internal tool and a global consumer service should not automatically use the same architecture.