Ozone:

Ozone is a gaseous molecule consisting of three atoms of oxygen (O₃), formed when oxygen molecules are split apart and rejoined again.  Ozone is a very powerful oxidizer and sanitizer.   It is a "free radical" that combines quickly with waste or turns back into common diatomic oxygen (O₂).

Ozone has been used for years in Europe to treat public water supplies.  It reacts faster than other sanitizers such as chlorine and bromine.  It is probably the perfect sanitizer for pools and spas.  It gives off a clean smell and taste and provides better water clarity because it contains NO residual.  Since there's no byproducts, there is no build up of Total Dissolved Solids like other products.

Ozone is a very strong oxidizer so it eliminates the need to periodically shock.  It will also regenerate bromine like other oxidizers.

Ozone is created naturally by either the suns ultraviolet radiation or by lightning.  It is created artificially by UV lights or electrical discharge over a grid and then introduced into the water.  Ozone is only introduced into the water when the ozone generator is in operation.

Since Ozone is a gas, it only lasts a few minutes in water.  It is also hard to maintain a consistent level of ozone in water.  Therefore, it needs to be supplemented with either chlorine or bromine to maintain sanitizing capability around the clock.