NodeJS is a JavaScript runtime that allows JavaScript to run on the server-side instead of only in the browser. It uses an event-driven, non-blocking model that makes it lightweight and efficient for building highly scalable web servers, web services, real-time apps, apps with queued inputs, and data streaming apps. NodeJS' package ecosystem npm is the largest in the world, and many companies like Netflix, Uber, PayPal, and Walmart use NodeJS in production. JavaScript's growing popularity also means NodeJS can be used in many other places beyond just web servers.