Below 35mm offers a wider perspective, suitable for landscapes. Above 40mm helps bring objects closer to the camera. A camera with a zoom lens offers a wide choice of perspectives as you zoom down or up. As the range of the zoom capability increases, the price of the camera tends to go up. Lenses available for an SLR offer many more choices.
Lenses are usually brand-specific.
If you have a Nikon SLR you need to use lenses by Nikon. If you have a Canon, Minolta, Pentax, Yashica or other brand SLR, similar rules apply. There are some other manufacturers that make lenses for these cameras, as well, but you must make sure that you're purchasing the lens for your appropriate camera mount. It adjusts the camera to optimal settings given the amount of light entering the lens. All you need to do is press the button for a perfect picture.