Nitrifying bacteria
The microorganisms responsible for the nitrogen cycle. They convert toxic ammonia into nitrites and then into less dangerous nitrates.
Detailed explanation
Nitrosomonas (and related genera) oxidise ammonia (NH3) to nitrite (NO2). Nitrobacter (and related genera) oxidise nitrites to nitrates (NO3).
These bacteria live on surfaces: ceramic filter media, sponges, rocks, substrate. They do not float in the water: this is why the filter material must be protected.
Commercial freeze-dried bacteria products (Dr. Tim's One and Only, Fritz TurboStart, Seachem Stability) contain concentrated cultures that accelerate cycling.
Nitrifying bacteria are aerobic: they need oxygen. A filter turned off for more than 2 hours kills them due to lack of oxygen.
Never wash filter material under the tap - chlorine kills them in seconds. Always use aquarium water for rinsing.