this one is a general basic zoom that lets you take pictures from the ultrawide (18mm) to the moderate tele (55mm) end. however, its max aperture is varied (gets worse by longer focal length) so it's nowhere near as bright at 50mm (5.6) as the 50/1.8. the latter lens is a fixed focal length lens (a so-called prime) where the zoom is your legs, but due to its brightness and large aperture it's better suited for low-light scenarios and when you need a shallow depth of field - nice subject/background separation.

You can only adjust the aperture in Aperture Priority or manual modes. It doesn't have an aperture ring like the D series because all changing in done inside of the camera modes. If you are saying that while you are in one of those modes that the aperture is not able to be adjusted and you suspect it is the lens, send it to Nikon so it can be checked.