The event loop is the heart of Node.js, but most explanations either oversimplify it ('just a queue') or drown you in libuv internals. The truth is in between: it's a series of phases, each with its own queue, processed in a specific order. Understanding this order explains why setTimeout(fn, 0) doesn't always fire before setImmediate(fn).

The Six Phases

Timers (setTimeout/setInterval), pending callbacks (deferred I/O errors), idle/prepare (internal), poll (retrieve new I/O events), check (setImmediate), close callbacks (socket.on('close')). Microtasks (Promise.then, process.nextTick) run between every phase transition, which is why they can starve the event loop if used carelessly.