Holding down a control key while pressing and releasing letter keys or certain other keys generates a "control character". E.g. holding control and hitting "A" generates control-A (ASCII code 1). The ASCII code for the control character is generally 64 less than that for the unmodified character.
Standard PC keyboards have two control keys, both labeled "Ctrl", at the bottom left and bottom right of the main block of keys.
The control key does not generate any character on its own but most modern keyboards and operating systems allow a program to tell whether each of the individual keys on the keyboard (including modifier keys) is pressed at any time.