Michael,
The DX sensor on a Nikon is smaller than a full frame or FX sensor. So in terms of 35mm equivalence, you multiply a lenses focal length by 1.5. This is the field of view you will have. The reason is the lens puts the same information you would have on an FX sensor in a smaller area. Sort of like cropping the picture and then blowing it up to the same size print if you get my meaning.
Canon actually has a smaller sensor and the factor is 1.6.
Anyway, all this only matters on a DX camera body or a DX lens on an FX body.
50mm lens would then translate to a 75mm equivalence or the 18-105 kit lens on your D90 would be 27-157.5MM.
Mark