Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. It allows JavaScript to be used for server-side scripting and provides APIs for networking and file system operations. Node.js uses an event-driven, non-blocking I/O model that makes it lightweight and efficient, allowing a single process to handle thousands of concurrent connections. It includes a package manager and common module system. Popular frameworks like Express make it easy to build scalable web servers and applications with Node.js.