Node.js is an open-source, cross-platform runtime environment that allows developers to run JavaScript code on the server side. Built on the V8 JavaScript engine, Node.js enables the creation of scalable and high-performance applications, particularly for real-time applications like chat systems or live updates. It uses a non-blocking, event-driven architecture, making it efficient for handling concurrent requests. Node.js is commonly used for building web servers, APIs, and microservices, and it has a rich ecosystem of libraries available through npm (Node Package Manager), which simplifies the development of full-stack applications.