A stock cooler is a CPU cooler that comes bundled with a processor when you purchase it. It usually consists of a small heatsink and fan attached to it. On the other hand, an AIO (all-in-one) cooler is a liquid cooling solution that combines a pump, radiator, and fan(s) into a single unit. AIO coolers are generally more efficient in cooling and can provide lower CPU temperatures compared to stock coolers, especially under sustained loads. They are often preferred for overclocking or for those who prioritize quieter operation.