A video adapter accepts raw video data from the CPU, processes that data, and supplies it to the monitor in a form that the monitor can display. In DOS text-mode days, that wasn't a demanding job. Early video adapters simply interfaced the CPU to the monitor, did little or no manipulation of the raw data, and depended on the CPU itself to render the data into a form usable by the monitor. When Windows arrived, the emphasis shifted from text mode to graphics mode, which increased video processing demands dramatically.
That made it impractical to use the CPU to perform video processing, and a new generation of video adapters, called graphics accelerators, was born. A graphics accelerator offloads the video processing burden from the main CPU by serving as a dedicated video coprocessor. In doing so, it not only frees up the main CPU, but also reduces the amount of video data that crosses the system bus, which also contributes to faster system performance. All modern video adapters are also graphics accelerators.
Formerly, all video adapters were separate expansion cards, a form in which they are still readily available today. However, demand for reduced costs has resulted in motherboards with embedded video circuitry becoming much more common, a trend that is likely to continue. Although they are inexpensive and tightly integrated, the problem with embedded video adapters is that upgrading the video may require replacing the motherboard. But all current video adapters are so good that anyone other than a hardcore gamer is likely to find them more than good enough to get the job done.