Consider a light turning on and off continuously once a second. Your eyes would see it going on and off. Now speed up the on-off cycles. When we get to the point where it is turning on and off about 60 times a second, the lamp would appear to be on continuously (although a little more dim). Less than that, and your eyes would still see that the light was going on & off, which we call "flicker".
TV pictures are repeated 50 times a second on the PAL system (actually, it's 50 half-pictures), but unfortunately some people still perceive flicker at 50Hz. NTSC mainly uses 60Hz (60 pictures a second), and people see a lot less flicker at this rate. The best way of observing flicker is to go to a big store with lots of CRT TV's on display, because the eye perceives flicker more at the edges of the visual field. So if you look at one of the screens, all the other screens will appear to flicker more. Because LCD and plasma screens generate pictures in a totally different way, flicker is a lot less of a problem. Some CRT TV's use 100Hz scanning, which basically shows each 50Hz picture twice, so eliminating flicker.