You have eight "Carpal" bones that form an arch in your wrist. If you look at your wrist as in the picture to the left, you will notice that it is rounded--that's your arch.
Notice that the top part of your wrist is hard--that's because it's the top of the arch made of bone. Feel the bottom of your wrist--notice that it's soft. That's due to the fact that there's no bone there. It's soft because your tendons, blood vessels and nerves travel under that arch.