A device is either a special software component (such as the Voice and Sound devices) or a piece of hardware (such as a joystick, mouse, or keyboard) that connects to your computer using a serial, ethernet, or other connection.
Devices react to either recorded data or live input from the device. An input device feeds data to MotionBuilder, and an output device receives data from MotionBuilder. For example, the keyboard and the mouse are input devices whereas the MIDI device is both, an input and output device.