Digital cameras use Charge-Couple-Device (CCD) technology where an optical sensor absorbs light. The larger the size of the CCD the more light is presented for absorption. The low to medium quality CCD sizes use up to 1/3 inch CCD while the more expensive units can be up to 1/2 inch or even more.
Also a camera with 3 CCD chips will take three different images and put them together to give you an arguable better quality image. I use the term arguable because there a couple of very high end CCD cameras that utilize only one CCD with very good results.
There is also a pixel count that basically equates to the more pixels the better the quality although some brand names have better quality in comparison with others at the same pixel count.
Getting a bit more technical there is also frame rate where a faster frame rate is needed for faster motion. The frame rate of 24 frames per second(fps) is the movie theater standard. They slowed it down in the old days to save on film stock.
There are other frame rates, three of which are related to regional television standards. They are NTSC, the North American Standard at 29.95 fps and PAL or Secam for Europeans and other parts of the world, at 25 fps.
These are interlaced standards. Video cameras can run in interlaced or progressive mode.
Progressive means that the whole image is taken at once. This is said to produce a better quality image or more like cinema.
Interlaced means two separate lines called fields are drawn per image, one set of lines drawn first and then another set of lines are drawn between the spaces of the first lines.
Due to the eye's ability to hold onto an image for a fraction of a second, the final picture appears as a single image. For stop action the picture is not good quality and progressive shots look better when stopped so it is popular for sports.
This is what you normally see from your TV.
For you real technical people PAL and SECAM use 625 refresh lines and for NTSC there are 525.
PAL is supposed to be a better viewing experience. The refresh frequency for NTSC is 59.94 Hz while PAL is 50 Hz. When you visit Europe you can almost see a flicker when watching TV.
For digital video there are emerging formats which are competing.
There is a difference between media and format which I will explain. Media is the physical medium used to store the material.
Flash Memory(Solid State, Memory Card, etc
Format is a more complicated topic and each format could be stored on one or more types of media. Also the formats are continually evolving so even these notes will have to be updated periodically.
1440X1080 The oldest HD format
1080i and 720p
MPEG-2 Transport Stream(JVC only)
The AVCHD is evolving and should be the most popular format in the future.
Camera's are now coming out with 1920 by 1080 resolution and even higher by some manufacturers.
You may have seen the numbers 4:2:0 or 4:4:4. These refer to compression of color. 4:4:4 means no compression. Below is a link from an expert.