Express is a minimalist and flexible web application framework for Node.js. It simplifies the process of building web applications and APIs by providing robust routing, middleware support, and easy integration with other libraries. Express is known for its lightweight architecture, giving developers full control over the application structure. It is widely used in the creation of RESTful APIs and single-page applications (SPAs). With its simplicity and scalability, Express is a go-to choice for developers working on fast, efficient, and maintainable server-side applications.