Video Cards are devices that display graphics and text on a monitor. They require special software called drivers to communicate with the operating system kernel. Different video cards differ in the amount of colors they can display (color depth), the amount of dots (pixels) per line, and the amount of lines per screen (pixels per line x lines per screen is referred to as resolution). At any given resolution and color depth, there is a speed at which each vertical and horizontal screen can be drawn called refresh rate or frequency (measured in Hertz [Hz]). Along with your video cards limitation to certain resolutions and refresh rates, your monitor must be compatible to display at the same setting.
Video cards use onboard RAM (Random Access Memory) to store the image before drawing it to the screen (buffering). The more RAM available, the larger the image (resolution) and the more colors available.
PC video cards are available for the ISA, PCI, and AGP interface. Macintosh video cards are generally available for the PCI and AGP interface.
Certain video cards are called accelerators in that their drivers are optimized to take special requests from the operating system and process them in much less time than a normal request would take. Examples are windows accelerators which draw windows components quickly, 3D accelerators which render and display polygons at tremendous speeds.
Performance tuning of Video cards includes overclocking the graphics processor, making it run faster and therefore increasing performance in situations where the video card is the limiting factor. When video cards are overclocked, they require extra cooling to ensure that they run without problems. New driver releases can also boost performance.