Here is an excerpt from Learning Maya 5 - Dynamics:

Maya provides two types of rendering for particles: software and hardware.

Hardware rendering uses the graphics buffer and graphics memory of your computer to draw the image to the display and then takes a smapshot of this image. This snapshot is then written to a file as a rendered image. This technique of using the hardware rendering capabilities of your computer has the advantage of being very fast but also the limitation of few rendering advantages such as shadows, reflections, and post-process effects like glow. [...]

Software rendering of particles will allow you to do post-process effects such as glow and incandescence as well as interactive effects of reflection, object occlusion, and shadows. The price for this functionality is time.

Hope this clears some...