Express is a minimal and unopinionated web application framework for Node.js, serving as the de facto standard for building backend services and APIs. It provides a thin layer of fundamental web application features, simplifying tasks like routing HTTP requests to specific handler functions and managing the request-response cycle. The framework's power lies in its middleware architecture, which allows developers to sequentially execute functions for tasks such as parsing data, authenticating users, and handling errors, offering a flexible and powerful foundation for any server-side project.