Hi Jamie
the difference is, the Sigma is considered a "D" type lens, which has an aperture ring at the back of the lens (near where it connects to the camera). This is useful when using the lens on a film based camera - for when you shoot in full manual
mode - to select your aperture.
The "G" type lenses are made for digital SLR's and do not have this aperture ring....as all dSLR's manipulate the aperture setting via controls on the body.
Hope that makes sense ??
Macro is the ability to photograph something at, or closer to, it's original size or scale.
For instance, if you zoom in on a flower for example from 10 feet away, you cannot completely fill the frame, so you move closer and closer. However, the lens has a "minimum focussing distance". Once you reach this, you cannot get any closer or you simply cannot focus.
A macro lens lets you get substantially closer, or shortens the "minimum focussing distance".
Of course, the macro
mode on the Sigma works ok, but for a true macro lens, you'd want to start looking at the Nikkor 105mm, which is a fully-fledged macro lens (ie. 1:1 scale - the Sigma's best is 1:2).
Possibly not the best explanation, but it's getting hard to think this late on a Friday