Every computer has a video card. It is needed to translate the information in the computer into an image which can be understood by a human. It shows your desktop, application graphical user interface, and everything you see on your screen. Most of the things you might use your computer for are 2 dimensional such as email, surfing the web, and running Microsoft Office. For these simple tasks only a simple video card is needed and will often be integrated into the motherboard, instead of on a separate card.
For 3d applications such as most games, or video rendering a better quality video card is needed to perform math and draw objects in 3d space. These high end graphics cards have powerful microprocessors, and memory on board the cards. The speed of the microprocessors and amount of on board memory are a few of the factors that affect their performance. These cards currently come in either AGP or more commonly now PCIeXpress.