A switchbox is used to channel different inputs to different outputs, and vice versa.
For example, a Parallel Port Switchbox will have a switch on the front, one input port on the back, and any number of outputs on the back. When the switch is placed on a certain selection, the output will pipe through to the cooresponding output port on the back, thus enabling several Parallel Port devices to share a single parallel port.
There are also KVM switchboxes, which enable one set of a Keyboard, Video Display, and Mouse to be used with several workstations.
There are limits to the KVM switchboxes I've used. When hooked through the switchbox, my monitor will not display any resolution greater than 800x600. Another quirk is the fact that the scroll wheel on the mouse will not be detected through a switchbox.