A graphics card is a device installed in a computer that consists of a graphics processing unit designed to help process and display images, especially 3D graphics. Graphics cards help take the processing strain off the main processor, and can contain their own memory to take the strain off the system RAM.
Onboard Cards
Onboard or integrated graphics cards are built into a computer's motherboard and are typically less powerful than cards installed in an expansion bay.
Dedicated Cards
Dedicated cards are graphics cards that are installed in PCI express, AGP, PCI, or other expansion bays, and have their own video memory separate from the system's main RAM.
Power Draw
High-end dedicated video cards tend to draw a lot of power from the power supply and may even require an extra power cable to hook up directly to the card.
Cooling
Most dedicated graphics cards have their own heat sink and fan right on top of the graphics processing unit to keep it cool. Proper case cooling is helpful to keep a graphics card from getting too hot.
Multiple Cards
Some computers can be installed with more than one graphics card. These computers can link two graphics cards to increase graphics processing power.