Fast lens and slow lens are probably what we often hear on the forums and all, and that annotation is basically related to the biggest diameter a particular lens can open to allow lights onto the image sensor. An idea of the relative size of the open diameter is shown on the above image. Now with a bigger aperture size, we get a shallower Depth of Field, which basically translates to images before and after our subject completely out of focus. With a bigger aperture, we can also shoot at a higher shutter speed, hence eliminating probability of a blurred image due to shake etc in low light conditions. The confidence to bring the ISO down below the law of averages can be there too. Obviously, lens with a bigger aperture size generally costs more with one exception - Nikon 50mm F/1.4D.
Do yourself a favor. Walk down to your Nikon store and get the Nikon 50mm F/1.4D (G if budget exists). It takes non-flash photography into the next level at less than $300 - a recommendation you would thank me for over and over again in months/years to come.