A system will always require what power connector?

Most computer systems, (providing that they aren't mobile devices like mobile phones), will use something like a kettle lead; those 3-pin blocky ones that you see.

Kettle leads are present on desktop computers, where they plug straight in to the PSU (power supply unit), while on laptops, they'd plug in to an A/C to D/C converter, the little cuboid box in between the wall socket and the laptop.