Express is a minimal and flexible Node.js web application framework that provides a robust set of features for building single-page, multi-page, and hybrid web applications. It is designed to make it easy to build fast and scalable network applications.

Express is built on top of the Node.js platform and uses its event-driven architecture and non-blocking I/O to handle multiple requests concurrently. It provides a simple and easy-to-use API for handling HTTP requests and responses, making it ideal for building RESTful APIs.

One of the key features of Express is its middleware architecture, which allows developers to write custom functions that can be executed for each incoming request. This allows for modular code and flexible handling of requests, making it easy to add new functionality to your application.

Express also provides a number of built-in middleware functions for common tasks, such as parsing incoming data, handling cookies and sessions, and serving static files. In addition, there is a large ecosystem of third-party plugins and middleware available, which makes it easy to add new functionality to your Express application.

In summary, Express is a fast, flexible, and easy-to-use framework for building web applications and APIs. Whether you're building a simple single-page application or a complex enterprise system, Express provides the tools you need to get the job done.