http://support.apple.com/kb/HT3470
Apple Mini DisplayPort video output port
Using the Mini DisplayPort video output port, you can mirror video or extend the size of the Mac OS X desktop (extended desktop mode) with a compatible DVI (Digital Visual Interface) display or VGA (Video Graphics Display) or projector using the appropriate Mini DisplayPort adapter:
For Single Link DVI displays, use the Mini DisplayPort to DVI Adapter (MB570Z/A).
For Dual-Link DVI displays, use the Mini DisplayPort to Dual-Link DVI Adapter (MB571Z/A).
For VGA displays or projectors use the Mini DisplayPort to VGA Display Adapter (MB572Z/A).