Most hydroponic systems are active, though some are passive.
Active hydroponics is the technique where nutrient solution is actively delivered to the plants roots via a pump.
No absorbent growing medium is necessary in active hydro; this allows an abundance of oxygen into the root zone. The constant supply of oxygen, water and nutrients leads to optimum growth and yields.
Passive hydroponics is the technique where nutrient solution is constantly made available to the plant via the capillary action of the growing medium surrounding the roots, there is no pump involved.
Although much more effective than traditional growing methods: passive hydro relies on absorbent growing medium and this limits the amount of oxygen in the root zone when compared with active hydroponics.