explain this to me - 4:3 and 720:480

Okay, VGA and standard television has an aspect ratio of 4:3 and 640x480 is 4:3. That makes sense.

So, why is DV footage and NTSC, which is 4:3 720 pixels by 480 pixels which is 3:2?

I would really appreciate knowing why there is 720 pixels in DV and NTSC, when the display is only 640 pixels wide.