What role do video cards play in a computer?

A video card's purpose is to support the use of graphics on a computer. It has two primary purposes, to output video signals to a supported device, such as a monitor or television, and to facilitate routines integral to placing visual data onto the attached device. Classic video cards provide only a "frame buffer", which is an area of system memory that is mapped to the video card's memory buffers; writing an image to this buffer will cause that image to appear on the monitor. Modern video cards also provide 3D rendering and memory for holding 3D objects such as textures. These cards are optimized for displaying 3D images in realtime without tying up the CPU for those operations.