Express.js is, in the words of the developers, a “fast, unopinionated, minimalist web framework for Node.js.”
Designed to be minimal and flexible, Express offers a set of features for creating web and mobile applications. From HTTP methods to built-in middleware, Express is designed to provide you with the features you need to build a web or mobile app on Node.js.
If you want to build an app on Node.js Express is a great choice, whether you use vanilla Express or one of the many frameworks either based on Express or built on top of Express. A few of these frameworks can be found here.