When you plug a projector (or second monitor) into your Macintosh laptop or computer for the first time it will probably display a different screen to your main display. To see the same on both screens you ned to turn on ‘mirroring’ in the displays preferences.
This is an Apple display adapter. They come in various shapes and sizes. The left hand end plugs into you laptop. There are 5 different possible left hand ends you can choose from: MiniVGA, DVI, Mini-DVI, MicroDVI and MiniDisplayPort! You’ll need to match it to your macbook. The right side plugs into a data-projector or monitor – there are 3 different right hand ends to choose from, VGA, DVI or HDMI. You’ll need to match this to your projector. If you always carry around the adapter to connect your mac to a VGA projector, that should get you out of trouble 99% of the time. If you want to be 100% sure, carry an adapter to connect to DVI as well.