This joystick module is very useful for controlling robots, for example. Connect the x and y to the analog pins of the Arduino and you are ready to go. The joystick also registers clicks, this port can be connected to a digital pin.
The voltage on the x and y pins on the module is half the input voltage when the joystick is at rest. When 5V is connected, both pins are therefore 2.5V. The voltage becomes higher or lower when the joystick is pushed up or down respectively.