CompuServe Messages

I've heard a lot about field rendering and how it makes animation look smoother, but can anybody explain to me exactly what's going on when you render to fields. I understand that an animation is rendered at 60 fields per second, where half the lines are rendered per frame. But how does this translate to video scan lines? I render at 640 x 480. NTSC is 525 lines, isn't it? What's the connection? How does 480 lines of pixels translate to 525 lines of video signal? Shouldn't we all be rendering at 640 x 525? This has always confused me.