Most TV sets don't show it because they overscan the signal onto the screen so you lose some information around all 4 sides, but above the lines that make up the TV picture is a header of 12 lines containing all sorts of digital-looking data patterns.
I captured the lines above from live TV and they're shown here magnified vertically 4x. I found that on the bottom line there is a 5-bit binary counter that goes from 0 to 29 over and over. (The animated GIF above is 60 frames, or 2 seconds, but it's probably not playing at the right speed due to limitations of the animated GIF.) But the rest is still a mystery to me. The closed captioning signals might be in there, but I haven't yet been able to detect any visual connection between closed captions and data in the header. Then again, I don't know what I'm looking for.
Also, the headers are different between channels and between programs. It seems like the top half of the header is always present while the bottom half depends on the program, so it might be present during a show but disappear for commercials for example. And the section in the middle having staggered lines of lengths that get shorter toward the right is present on some channels but not others.
I'd love to understand what all this stuff means, but I can't find any sites about it. Looking up info on NTSC standards only shows me all the electronic stuff behind the signal that goes over the cable, talking about voltages and stuff. So I'm guessing this header might not be so much a part of the NTSC standard but maybe more of something that TV stations and programs insert on their own to mark video signals with useful information. In either case, I just want to know what their codes mean. :)