
Every computer has a graphics card. Yes, even Macs and laptops. A graphics card is what pushes your video signal to your monitor (obviously, gosh). The better the graphics card the better the graphics you can push out of your machine. Since most people just need a flat 2D desktop space to run their programs they usually get a pretty tame graphics card. If you want to run some cool games you need to be able to do some cool 3D processing and that's why you get a pimped out GC (graphics card) with extra memory, dual DVI outputs (two monitors), and a fan to keep the 'lectronics cool. They can cost from $50 to $600. I bought one online for $100.
You also need a pretty good computer (PC) with a fast processor and lots of RAM to push out some cool graphics (w/ the help of the GC). 99% of all the cool games are Windows only so running a Mac pretty much means you can't run super-high-tech games but World of Warcraft runs on a Mac.