Most of you know what pixels are. In a sense, film has pixels. In actuality, those are tiny particles that change chemically when light strikes them. The particles are too tiny to see with your eye, so when you look at the picture you see the composite image made by all those tiny bits together. A video pickup works somewhat the same way.
Light strikes the pickup device, effecting thousands of tiny light sensitive spots.
The brighter the light, the more charge is held by a particular pixel. Professional video cameras usually use video tubes to pickup the image. Almost always there are three of them - one for each of the primary colors. Home video cameras have a single pickup designed and adjusted to pick up all colors in the proper proportions. The light sensitive material is usually either Saticon or Newvicon.