The D-pad, also known as the Directional Pad, is a form of directional control first implemented on the Game and Watch unit Donkey Kong, and featured prominently on the Famicom (NES). It includes four different directions including up, down, left and right. The direction you press will cause the object on screen to move in that direction in most cases. For example, in Super Mario Bros. for the NES, if the player presses right on the D-pad, then either Mario or Luigi (depending on the character) will move in that direction.
Most recently the d-pad, while still found on most controllers, has been replaced by the modern day Joystick, which was first featured on the Nintendo 64.
Unlike most controllers, the Virtual Boy had two D-Pads. One for moving around with and to look around.