Node.js is a platform for building server-side applications using JavaScript. It allows JavaScript code to run outside of a browser, using an event-driven, non-blocking I/O model that makes it lightweight and efficient. Node.js applications are written in JavaScript and use callbacks and streams for asynchronous programming. Common uses of Node.js include real-time web applications, IoT applications, REST APIs, and more. The Node.js community is large and active, with many open source modules available on npm.