Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for building web and mobile applications. As the de facto standard for building web servers in Node.js, Express simplifies the process of creating APIs and web applications by providing a thin layer of fundamental web functionalities, such as routing, middleware, and templating. This modularity allows developers to add only the components they need, keeping their applications lean and efficient.

Express is known for its simplicity and unopinionated nature, meaning it does not enforce any particular structure or style, giving developers the freedom to design their applications in the way that best fits their needs. Its wide adoption has also led to a rich ecosystem of middleware and plugins that extend its capabilities, enabling everything from authentication to input validation. 

By using Express, developers can build scalable, high-performance web applications with minimal effort, leveraging its integration with various databases, template engines, and other tools. This flexibility and ease of use make Express.js an ideal choice for both small projects and large-scale enterprise applications, where rapid development and maintainability are crucial.