There are several 50mm lenses. This one is a blend of fast aperture (F1.4) and price $330 (Nikon makes the same lens with manual focus override for $550). The 50mm F1.8 is also a great lens for around $125 and does almost everything that this lens does... except see in the dark.
This is called the "normal" lens because the images you print look "normal" and without distortion to the naked eye. While the term can have a general application for a range of focal lengths that make images appear "normal," it has become a short hand way of describing the popular 50mm lenses typically used on a 35mm camera.
The only reason I have the F1.4 version, is for my work in hospitals with newborns. When compared with other venues, hospitals have smaller spaces, less light and babies don't like flash. Unless you are going to do a lot of photography in dimly lit rooms, I suggest getting the "Nifty 50." Even at F2.5, the background is sufficiently soft.
Newborn "C" at 20 hours