Node.js is an open-source JavaScript runtime built on Chrome's V8 JavaScript engine. It allows developers to use JavaScript for server-side scripting, enabling the creation of scalable and efficient web applications. Unlike traditional server-side programming languages, Node.js uses an event-driven, non-blocking I/O model, which makes it lightweight and ideal for building fast, real-time applications like chat apps and live data streaming services.

Node.js allows developers to build full-stack applications using JavaScript on both the client and server sides. Its extensive package ecosystem, available through npm (Node Package Manager), provides access to a vast number of modules and libraries for different functionalities. Node.js is highly suited for building microservices, RESTful APIs, and single-page applications, and it has gained popularity in the tech industry for its scalability and performance.