Node.js is a JavaScript runtime built on Chrome's V8 engine that uses an event-driven, non-blocking I/O model to build fast, scalable network applications like real-time chat servers and file upload clients. It is well-suited for data-intensive real-time applications because JavaScript has no threads and uses an event-based concurrency model. Node.js allows building full-stack JavaScript applications using the MEAN stack with MongoDB, Express, AngularJS, and Node.js itself.