A Joystick is a device that usually has a stick pointing up which you move left, right, up and down and all points in between. When you do that, inside there are sensors or what are called "variable resistors" that send different values as information to the computer. These different values are interpreted as a location.

So, if you were playing a video game, pressing the joystick forward may tell the computer that you are moving forward in a video game. Early joysticks were big. clumsy and often only had one or two buttons that would tell the computer to do something else. Like fire a Phaser at an alien in a video game. Ask your parents about Space Invaders!

Todays joysticks have evolved into complicated modules like an X-Box 360 controller which has joysticks but your use your thumbs because the stick part is not there anymore. Even the Wii controller is a type of wireless joystick.