Node.js is a single-threaded, non-blocking, event-driven JavaScript runtime that allows for JavaScript execution on the server side, outside the browser. Central to its operation is the event loop, a semi-infinite process that handles asynchronous tasks without blocking the main thread, thus ensuring efficient performance. The event loop consists of six major phases: timers, pending callbacks, idle and prepare, poll, check, and close, each managing different types of asynchronous operations. While JavaScript runs on a single thread, Node.js utilizes a thread pool to handle computationally expensive tasks concurrently, avoiding performance bottlenecks. Understanding the event loop is crucial for writing robust, non-blocking Node.js applications, as it helps developers anticipate and manage the execution order of asynchronous callbacks, microtasks, and "next tick" tasks, thereby optimizing server responsiveness.