A queue that holds messages (callbacks) to be processed. When the call stack is empty, the event loop picks functions from this queue to execute, enabling asynchronous behavior in JavaScript. • **Macro Task Queue**: Holds tasks like setTimeout, setInterval, and I/O operations. These tasks are executed in a first-in-first-out (FIFO) order after the current stack is cleared. • **Micro Task Queue**: Contains tasks such as promise resolutions and process.nextTick() callbacks. Microtasks have higher priority and are executed immediately after the current operation completes and before any macrotasks.