An integral part of military demolitions. This is the bit that goes bang first and makes everything else go bang, but much louder.
Detonators come (for the military at least) in 2 varieties:
Electric: A small but powerful charge is set off by an electric current completing a circuit.
Non-electric: A small charge is set off by heat/flame, usually by safety fuse, which is the stuff you see in movies smoking and burning. A handy way to remember how much time you have is to cut a length the same as what you are using with your detonator, and light them at the same time, holding the second bit of fuse in your hand. When your fingers get warm, it's time to move away sharpish.