Here is a link to the Sigma 105 mm macro lens
http://www.bhphotovideo.com/c/product/341922-USA/Sigma_257306_105mm_f_2_8_EX_Macro.html
And here is the Nikon version, with VR (vibration reduction build into the lens).
http://www.bhphotovideo.com/c/product/424744-USA/Nikon_2160_105mm_f_2_8G_ED_IF_AF_S.html
As you can see the Sigma is considerably cheaper, but still a good lens.
These are both autofocus lenses, and will also allow manual focusing. For macro photos sometime manual focus works better.
The other number 2.8 relates to the aperture opening. With apertures, smaller the number means larger the aperture, and hence lets more light in. So a f22 would be a small aperture and F2.8 would be a larger aperture. The aperture is also important in controlling the Depth of Field or Depth of focus. When you take picutures, there is a certain range of distance within which the picture will be in focus. This is called depth of focus. Anything else in front or behind it will be out of focus and blurry. The smaller the aperture (ie larger F number) the larger the depth of focus you can get.
And ofcourse the smaller the aperture, the less light reaches the sensor. You can compensate for that in several ways, each with its own trade offs.
1) use a flash... but for macros the overhead flash or camera flash may not work well since you are too close to your subject and the light gets blocked by the camera.
2) use a slower shutter speed. This lets more light reach the sensor, but at slow speeds its almost impossible to hold the camera steady, hence the need for a good tripod.
3) Use a higher film speed setting - called ISO number. The trade off here is that with higher ISO number, your picture will get grainier. Ofcourse this is lately not true with the new Nikon D3 camera.
So as you can see you have several controls (aperture, shutter speed, and ISO settings, flash/no flash) to control the light and each comes with its own limitations and constraints.
sanjay.