Node.js is an open-source, cross-platform JavaScript runtime environment that executes JavaScript code outside of a web browser. It allows developers to run JavaScript on the server-side to create fast, scalable network applications. Node.js uses an event-driven, non-blocking I/O model, which makes it lightweight and efficient, perfect for data-intensive real-time applications that run across distributed devices. Node.js also has a large ecosystem of open-source libraries, making it easy to develop and maintain web applications.