The Alt key is a modifier key typically found on either side of the spacebar key. It is used in key combinations, much like the Shift key. The instructions executed by the key combination usually depend on the application currently being run.
The most common key combination including the Alt key is Ctrl + Alt + Del. The combination is used to either open the task manager window or reboot the computer. In Windows based computers, users can type in special characters by holding down the Alt key while typing in a specified series of numbers on the numeric key pad. The Alt key is not present on all computer keyboards. On Macs, the Alt key has been substituted with the Option key.