Cropping

see http://www.uwasa.fi/~f76998/video/conversion/ 

Cropping is where a few lines at the top and bottom of the screen are cut off from view.  The number of lines that are cropped varies from TV to TV, but is usally anywhere from 4 to 6 lines.  This also explains why many sources claim that there are 480 visible scan lines.  There are 486 active scan lines but after cropping, only 480 visible scan lines are left.

The 525/59.94 video signal has 486 active (image-carrying) scanlines, but modern digital video equipment usually crops off 6 of them. Why? To get the height of the image down to 480 pixels, which is neatly divisible by 16. See for yourself:

Also note that 720 / 16 equals exactly to 45 so the width of the image is divisible by 16, as well!

Why is it important to have the height and width of the raster image divisible by 16?

Modern digital video applications such as DV, DVD and digital television (DVB, ATSC) often use MPEG-1 or MPEG-2 formats (or their derivatives) which are all based on 16×16 pixel macroblocks. Having the height and width of the image readily divisible by 16 makes it easier and more efficient for an MPEG encoder to compress video.

Doesn't this mean that when capturing in 720×480, I will lose six scanlines worth of valuable information that was once present in the original video signal?

Correct, but the information might not have been that valuable in the first place. Most 525/59.94 video work is already done solely in the digital domain and in the 720×480 format, so there is usually nothing to digitize on those scanlines anymore. Moreover, in the good old days (when all of those 486 scanlines were still in active use) most of the time the edges only carried flickering VCR head noise.

The video image is masked by the overscan edges of a CRT based television, so you would not normally see the "missing" scanlines, anyway.