Node.js is a powerful, open-source, cross-platform JavaScript runtime environment that is built on Chrome’s V8 JavaScript engine. It is used to create fast, efficient, and scalable network applications. What makes Node.js fast is its asynchronous, event-driven model, single-threaded model, non-blocking I/O, and lightweight runtime environment. All of these features work together to make Node.js an incredibly fast and efficient platform for building real-time applications.