It is a NodeJS web framework that is used in frameworks like MEAN (MongoDB, Express.js, AngularJS, and Node.js) and MERN (MongoDB, Express.js, ReactJS, and NodeJS) stacks. This framework provides numerous benefits in web development and is used in creating high-performance web apps. This framework is popular for its great performance and minimalistic design. Apart from the minimalistic design, it provides numerous features like easier management of routing, plugin integration in server-side code, and middleware packages. You can also create APIs with Express.js to integrate with other web apps.