There are tons of video cards out there to choose from, Also known as Display Adapters or Video Adapters. Their have been two trends in Video Cards these days, with the high end Video Cards getting increasing more powerful and versatile, and The low end cards becoming simpler and more integrated. What type of Video Card you need depends on what type of applications you will be using your computer for. If you need your computer for basic home use with not too many 3D games or Multimedia applications called for, then a good moderately priced Video Adapter should more than suit your needs. As a matter of fact the ones built into your Motherboard can often offer you significant performance advantages. If you are a serious Gamer however,  then you should look towards a higher end Video card. 

As for power, consider what you'll be doing with the PC. If you're doing mostly business and internet and the occasional game, then you don't need a gaming card with a GeForce processor, for example.  On the other end, if you're into performance  then get yourself a high end graphic card. 

The video card is the basis of your display. It contains all the circuitry relating to your computer's ability to send a picture to the monitor. It usually is a separate card that fits into one of your motherboard's slots, but sometimes this circuitry is present on the motherboard itself. 

When buying a new video card, it is best viewed with a good monitor. If you've got a fancy new video card and a puny monitor, you just won't be able to view the wonders of your card that you paid good money for. Make sure you have a monitor that can handle what you get. There is no sense in buying a kick-ass video card with awesome refresh rate if you're using an old monitor that can't handle it.