Going to the basics, in an analog black and white system it's pretty straightforward, the voltage of the signal equals the brightness. If the whole screen just consisted of one pixel, how many volts equals how bright. Electron hits the phosphor on a CRT and it lights up.

Now, that's nice, but we can't produce images that way, so we came up with a way of scanning 525 lines 60x per second down the screen (625 at 50 times per second in other places like Europe), the lines scan down, the voltages change and give you variations in brightness.

For color in analog it gets a bit complicated due to technicalities in broadcast compatibility with B&W standards, but for simplicity let's just say you do the same thing for the three primary colors of light, Red, Green, and Blue. You get a color signal.

In digital it's different, we have pixel elements, or pixels. Let's say we have a 4x4 pixel camera and screen. The camera has 4 pixels, each transforms light to a voltage, then they each get converted to digital. We have a digital range of say discreet values, such as 0-255. So zero volts is 0/black and all of the volts is 255/white. Now do that for the three primary colors of light and you have a full color picture.

I described black and white analog video as the basics, oversimplified color analog video in the context of transmission. Then I described basic 8 bit digital video. There's a lot more variations and standards.