Node.js is a JavaScript runtime environment that allows JavaScript to be run on the server side. It uses an event-driven, non-blocking I/O model that makes it lightweight and efficient for data-intensive real-time applications that require persistent connections. Some common uses of Node.js include real-time web applications, building APIs, and handling multiple connections at once without creating new threads. The document discusses why Node.js is well-suited for applications that require maintaining persistent connections from the browser to send updates in real-time using techniques like long-polling.