MacBook Pro Display Adapter
The Output port from early MacBook (and MacBook Pros) was a mini-VGA, later a mini-DVI port and various accessories were available to connect regular size VGA displays. 15" and 17" PowerBooks had full-size DVI outputs as far as I can remember.

Recent MacBook models use a smaller mini DisplayPort. Apple itself offers different adapters to transform the mini DisplayPort into VGA, DVI, or HDMI. If you need a VGA adapter (like you say in your question tag), you are looking for this. If, on the other hand, you need to connect a DVI display, then this is what you want.