Express.js is a minimal and flexible Node.js web framework used for building fast and scalable web applications and APIs. It simplifies backend development by providing a robust set of features for routing, middleware management, and HTTP handling.

Express.js is known for its lightweight and unopinionated nature, allowing developers to structure applications as needed. It offers a powerful routing system, making it easy to handle URL paths and API requests. Middleware support enables request processing, authentication, logging, and error handling, enhancing functionality. As it runs on Node.js, it benefits from asynchronous, non-blocking operations, making it ideal for real-time applications like chat apps and dashboards. It also integrates seamlessly with databases like MongoDB and PostgreSQL and supports templating engines for dynamic web pages. With a vast ecosystem of third-party middleware and compatibility with frontend frameworks like React, Angular, and Vue, Express.js is a go-to choice for building RESTful APIs and full-stack applications.