To cram an entire computer onto a circuit board or in a thumb drive, you have to give up a few features. One of those is a cooling system. A circuit board or thumb drive can't accommodate a fan or a water-cooling rig. And that can be a problem -- computing generates heat. That's because computing relies upon electricity and our methods of harnessing electricity aren't perfect. We always lose some energy in the form of heat -- wires and connections heat up as electricity flows through them. With too much heat, a system can break down -- pathways expand, connections break and the computer stops working.
That's one reason most of these computers use ARM-based processors. An ARM-based processor is ideal for small, mobile devices. They're small and efficient. They may not measure up to the processing speeds of a state-of-the-art CPU, but they can still pack a data-crunching punch.
Many of these small PCs also lack a real-time clock (RTC). The RTC is the timekeeping device on your computer that keeps going even after you power down. That's why your computer's clock keeps time whether the entire computer is on or not. The RTC pulls power from a dedicated battery. But while engineers have reduced the size of components like memory and processors, battery technology hasn't kept pace. A battery would add more bulk and heat to the system, and so a mini PC may not include one.
Perhaps the most obvious missing elements of a mini PC are the physical interfaces we rely upon to input and receive data from a computer. This includes a display and interface like a keyboard, mouse, track pad or touch screen. Some mini PCs support the Bluetooth standard, allowing you to use Bluetooth peripherals. Otherwise, you may need a USB hub to connect your accessories to a mini PC.