Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It sits atop Node.js as a de facto standard server framework for building REST APIs, web applications, and microservices. While Express is unopinionated and lightweight—leaving architectural decisions to developers—this flexibility becomes its greatest strength when building scalable server-side applications. Understanding middleware execution order, asynchronous error handling patterns, and the request-response lifecycle is essential for leveraging Express effectively in production environments.