A VGA Splitter is a device that allows one PC to split its VGA signal through multiple VGA connections. This technique is useful for displaying the output from one PC to VGA devices such as monitors. You can get 2 types of VGA splitter, internal and external.
The external version is a hub type device accepting an incoming connection from a PC graphics card and then, depending on whether it is a 2, 4, or 8 port splitter, allows the connection of 2, 4, or 8 monitors.
The internal version is a card that fits into a PC, taking the place of 1 PCI card, but does not physically attach to the main board. Again this device takes the input via means of the computers graphics card and then outputs to the connected monitors. As well as VGA these splitters are also available for DVI Monitors.