A computer callous is located on the heel of your hand, usually the hand you use to move your mouse.
It is usually caused by the heel of your hand being rubbed against the mouse mat for long periods of time, usually during all nighters on the internet.
A computer callous is most often a little more red than your usual skin tone and is quite soft and smooth.
Hoshit, I've been on the computer so long I've aquired a computer callous.