"Full" is the "Stretch" mode you refer to. This is for anamorphic DVD's -- it stretches 4:3 video horizontally to fill the 16:9 window. The video is not altered vertically.
If you have an interlaced DVD player, and watch enhanced/anamorphic DVD's, they are presented in the correct aspect ratio when you set the TV to "full" mode. Many folks who don't like the gray side bars stretch all 4:3 material to fill the screen this way -- after a while, you get used to the distortion.
"Fill" is the "zoom" mode you refer to. This is for letterboxed/matted DVD's and television shows (like ER). This mode expands the video horizontally and vertically to fill the letterboxed video on the screen. You can also use it to zoom 4:3 material while keeping the correct aspect ratio -- if you have a good signal, baseball is nice zoomed in this mode.
If you have a progressive DVD player, the screen modes do not work. You need to get a DVD player that performs scaling. Do some searches on the DVD hardware forum for info on scaling DVD players.
Joe