I would like to use my MacBook Pro with a second monitor. What do I need to be able to do this?
Your MacBook Pro could have various display ports depending on its vintage:
Both can drive a monitor in analog mode (via a VGA port on your monitor) or in digital mode (via a DVI port). Most monitors, except really ancient ones, have both connectors. The quality of the image beeing better with a DVI connection I recommend you go this route.
All monitors are shipped with a DVI/DVI cable, if not go get one at your nearest cheap PC place. For old MacBook Pro that's all you need. For more recent models you need to add a adapter. Apple has one, but I don't like it: It's really short and put pressure on the port when the computer is used on a support. Not to mention ugly. I bought on ebay a 6 foot Mini DisplayPort/DVI cable for cheap, it's all digital so you don't need huge shielding and "super copper wire balanced to give you the best image". Around 10 quids.
Note: Smaller models like the MacBook and the Macbook Air came with different ports: A mini DVI or a micro DVI. For the people reading this with a non "pro" model, make sure you check for those as well...
PS: You can find pictures of the various ports by searching for their names on Wikipedia. I wanted to put the links directly, but apparently I do not have enough reputation to post more than one link:(
What MacBook Pro do you have?
Chances are good that all you'll need is:
If the monitor has DVI in, that adapter needs to be Mini DisplayPort to DVI. If VGA, it needs to be Mini DisplayPort to VGA—and so on. But that should be all you need.
Edited to add:
Apple has some good pages:
They should help you identify what you have and what you need.