VGA stands for video graphics array and HD stands for high-definition. Although both terms are widely used for other applications in the video world, when you hear them referring to cameras, they usually mean those with old technology versus those with new.
Developed in 1987, VGA originally had a resolution of 640 by 480 pixels to illuminate dots on a screen. As time passed, technological improvements increased resolution in various stages to the high-definition quality we see in widescreens today, but with different letter designations ending in "GA." In most applications, the term VGA has come to mean a classification that includes all of those resolutions. But in video camera jargon, it only refers to those with the original 640 by 480 pixels of resolution.
Technically speaking, high-definition refers to any signal higher than 640 by 480 pixels, but most commonly refers to resolutions of 1,280 by 720 and 1,920 by 1,080 used in high-definition television signals. For simplicity, resolution specifications are shortened to use only the vertical or second number, and in video cameras, may be defined as lines of resolution. A high-definition camera may only have 480 lines of resolution, but may produce more information in the form of mega-pixels to improve the overall image quality. Such cameras may be classified as HD, even though they don't produce a true HD signal.
Mega-pixel ratings for cameras may refer to the number of elements that pick up the color signals on the video sensor and not the signal it sends to a monitor. References to a VGA camera do not use the term, but the total number of pixels a VGA camera produces is equal to the vertical resolution times the horizontal, or 640 times 480 or 307, 200. This is only .3 mega-pixels, a relatively small number compared to the high mega-pixel ratings of many cameras today. Presently any camera that produces more than the .3 mega-pixels of a VGA camera can technically be called HD.
Camera Quality Considerations
The megapixel rating of the sensing device in a camera only tells part of the story. A larger sensing device can pick up more informaiton and record it faster than a smaller one, even though its megapixel rating is lower. A larger and higher quality lens presents a clearer image to the sensing device than a small lens, and ultimately produces a higher quality screen presentation. A camera balances lens quality with sensor size and megapixels ratings along with other features such as automatic settings and sensitivity in low light to determine its overall quality.