Difference between DX & FX Nikon?

I don't know what the difference? Full Frame?

DX - Designation for Nikon's APS-C sensor DSLRs and lenses designed to be used only with APS-C bodies. DX lenses can be used on FX bodies, at reduced resolution (usually half resolution - The D3, D700 and D3s can use DX lenses at 7MP). DX bodies and lenses are less expensive and are fantastic for beginners.

FX - Designation for Nikon's Full Frame sensors and lenses designed for FF cameras. FX lenses are 100% compatible with DX bodies. FX bodies & lenses are more expensive than DX bodies & lenses.

FX - Full Frame
DX - Smaller format, APS-C, 1.6x crop factor

FX is basically as big as film, 36 x 24mm, while DX is ~1.6x smaller, thus crops the image 1.6x.

There are certain lenses that are specifically made for DX cameras that will not work that well on FX. FX also has less noise and better quality most of the time.

Here's a DX and FX Explanation - http://www.the-dslr-photographer.com/2011/09/nikon-dx-and-fx-sensors-explained/

Sorry Eric;

We're talking Nikon here; Nikon DX crop factor is 1.5x.

Canon crop factor for their APS DSLRs is 1.6x.

Thus, Nikon DX cameras have slightly larger sensors than do Canon cropped cameras.

But in reality, the difference between 1.5x and 1.6x is not that significant, but that is probably why Nikons usually have slightly better low-light performance than does Canon.