wow, ok i'll be more specific.
Light comes in through your lens, and the image sensor converts the light and color information it receives into an electrical signal that then gets digitized and transcoded onto the magnetic tape. There are several components to the video signal, mainly luminance and the three colors (red, green, blue) and sync. The information is recorded digitally, dont think one's and zero's, think on/off switches. It's all based on an electrical current. Since we're talking mostly about the luminance value of the signal, having to do with noise, we'll just focus mainly on that. I assume you know about Fstops, which represent the latitude in which your device can measure the light information. Anything (depending on your camera/settings) under 0 and over 100-110 IRE clips and is either blown out (over exposed) or completely black. Over exposure, and black have no detail in them because there was not enough, or too much light to record it. Noise is that lack of detai when you electrically boost the signal to bring the luminance value up.
That's all just straight off the top of my head, i could go back and check all the notes I've taken in classes, or do some internet research but I thin i've explained it pretty thoroughly. Noise is electricity. It looks grainy because the imager could not record detail in that area, but it was brightened so as not to be completely in shadow so the grain becomes the detail.
If you'd like me to explain the noise you sometimes get when shooting something moving with a lot of colors, such as fall leaves blowing in the wind, I can do that too. Unless you know all about color space, bit depth, sampling ratios etc.
