[spanko]

Okay here I go.

Ammonia produced by decaying organic matter, consumed by aerobic bacteria with the resultant byproduct of nitrite.

Nitrite consumed by another aerobic bacteria that results in the byproduct nitrate.

Now it gets sticky for us marine aquarists.

Nitrate is consumed by an anaerobic bacteria with a byproduct of nitrogen gas.

Where it gets sticky is the aquarists ability to provide enough anaerobic area for enough bacteria to reside in to convert the nitrates into nitrogen gas. In most aquariums this is not able to be done. The amount of rock to provide enough area free from oxygen (anaerobic) is mostly not practical. A DSB is also a way to provide the anaerobic zone but again most aquarists don't understand DSB's and how to correctly set them up, as well as the drama in the community of the affect\effect of DSB's long term.
So I guess where I am coming from here is Live rock and sand in a sump or fuge are just more areas to house aerobic bacteria. The best way for the marine "hobbyist" to reduce nitrate is through:
Mechanical filtration which removes the organic matter before it becomes ammonia.
Water changes that reduce the nitrate by dilution.
Skimmers that remove organics before they become ammonia.

Remember, live rock and bio-media do not collect nitrates, they collect organic materials that in turn become nitrate some where down the road. This is why people turkey baste their rocks, clean their bio media, and try to get detritus (organic matter) out of the system before it becomes a problem.

The above is my understanding of the way it works.