Express.js is a free and open-source web application framework for Node.js, which is used to build web applications and APIs. It is built on top of the Node.js platform, and it provides a minimal and flexible way to create web applications and APIs.
Express.js provides a set of features for web and mobile applications, such as routing, middleware, and template engines. It also has a variety of built-in middleware functions, such as body-parser, which can be used to parse incoming request bodies, and cors, which can be used to enable cross-origin resource sharing.
Express.js also has support for template engines, such as Pug and EJS, which can be used to create dynamic views for web applications.
Express.js is a popular choice for building web applications and APIs because of its minimal and flexible design, making it easy to get started and add new features. It also has a large and active community, which provides a wide range of resources and tutorials for learning and using the framework.
In summary, Express.js is a web application framework for Node.js, it provides a minimal and flexible way to create web applications and APIs, it includes routing, middleware, template engines and has a large and active community.