The difference between a diffuser downpipe and a normal glass downpipe is the way that the smoke is released after it is pulled down the glass downstem. The diffuser downstems include an average of 6 slits which are placed at the bottom, allowing the smoke to exit via multiple angles.
The benefits of this is that the smoke is separated giving more surface area to be cooled by the water inside the bong, it can make a huge difference and I always recommend the diffuser downstem above the normal glass stems.
Diffuser Downstem for Bongs
The diffuser downstem is placed inside the bong and apart from the previously mentioned slits at the bottom, it looks exactly like a normal glass bong downstem. The price is pretty much the same too, which is a huge bonus for the added cooling that you receive from using one.
Diffusers have been around for a good five years now and came before percolators, which were pretty much styled on the diffusers method of giving an increased surface area on the smoke.