Node.js is an open-source, cross-platform JavaScript runtime environment that allows developers to run JavaScript code on the server side. Built on the V8 JavaScript engine, Node.js is designed for building scalable and high-performance network applications. It uses an event-driven, non-blocking I/O model, which makes it efficient and suitable for real-time applications like chat, streaming services, and web servers. Node.js also has a rich ecosystem of libraries and modules available through its package manager, npm (Node Package Manager), enabling developers to extend its functionality easily.