The graphics card can be related as the retinas in your eyes. They are an optional expansion card that connect into a PCI x16 slot in the motherboard, and generates a feed of output images to a display (such as a computer monitor). Only reason a person would need this optional expansion card is for running graphics intensive programs such as 3D modeling, or video games. The more powerful the graphics card is, the more clear and smooth those programs will run. A very powerful card can even power and run more than one monitor. Virtually all current video cards are built with either AMD-sourced or Nvidia-sourced graphics chips. There is minimal difference between these two companies other than the fact they are competing in the same market.
AMD-Sourced Graphics Card Nvidia-Sourced Graphics Card
If the motherboard has more than one PCI x16 slot, then you can have more than once graphics card in your computer. This is known as CrossFire(AMD) or SLI(Nvidia). This will double their power and requires a CrossFire Bridge or SLI Bridge that connects the more than one graphics cards together. These Bridges generally come with the purchase of SLI or CrossFire compatible motherboards.
