Caps Lock is a button on a computer keyboard that, when pressed, causes all letters to be generated in capitals until deactivated. It is located in the position of a similar Shift lock key (and sometimes a Ctrl key) found in some other keyboard layouts. It is usually a toggle key: each press reverses its action.
Exactly what Caps Lock does depends on the operating system and driver, and the keyboard layout implemented. On non-IBM PC-compatible computers it may also depend on the keyboard hardware. Usually the effect is the same as pressing keys with the shift key depressed; letters are capitalised, and non-letter characters are generated as if the shift key were depressed. For example, on most keyboards the "5" key generates "5" in non-shift mode and "%" if the shift key is pressed. If caps lock is activated the key generates "%" until deactivated. In other cases the key may only affect letters, so that the "5" key always generates "5" unless the shift key is pressed.