The cutoff?
projector lenses have a clearly defined cutoff, i.e. a horizontal line below which the light is cast, and above which hardly any light is cast. this keeps more light on the road, and out of the eyes of oncoming drivers. usually, the sharper and more defined this cutoff is, the higher the quality of the projector. you want to avoid a projector that has "hotspots," or areas of light above the cutoff, as this may shine directly in the eyes of oncoming drivers
Different projectors also have different widths. some may cast light only to one lane on each side of you, while others, like the honda s2000 ones I have in my car, will light up a 6 lane highway side to side.
http://customlightz.com/?p=p_174&iPageContent=2 some comparisons, although these are HID projectors, but it should give you an idea what i'm talking about.