The
key on the far left of a
keyboard, just above
Shift, just below
tab. It serves to "
lock" the alpha keys into
uppercase.
I consider this to be largely a holdover from manual typewriter days when keys were actually physically difficult to depress and it would be a boon to anyone working primarily in capital letters to not have to exert themselves to hold down two heavy keys for every stroke.
Currently it serves to make shouting in chat rooms that much easier. This is a bad thing.
czeano informs me that the position above originally described the control key - it wasn't until the advent of the PC that manufacturers gave caps lock the more prominent placement.