Paintball guns are powered by a Air and Co2 bottle that fits into the back of the gun. An often asked question is the difference between Air and Co2. So where do we begin?

Air is also called Nitrogen, the advantage it is very stable and is not affected by outside temperatures. The disadvantage is the bottles are more expensive and to get the number of shots you need much  higher pressure than Co2. You'll see on a air system or tank figures such as 3000 PSI 48cu.
The 3000 relates to the amount of pressure the tank can hold and the 48 relates to the capacity inside the tank. 
The system or tank can be brought either in aluminium or fibre wrap.
The aluminium is cheaper but heavier, the fibre is the opposite, dearer but lighter. The aluminium does not require a cover, the fibre wrap does. 
To fill the air systems or tanks requires a scuba air fill station which attaches to a scuba tank which one can purchase from any scuba shop or second hand shop like cash converters when they're available . Air systems

Co2's advantage is that its cheaper to buy at the start. The bottles are less than half the price of Air. That is its only advantage. The gas is not as stable as air due to the bottle is filled with both liquid and gas Co2 and relies on the liquid turning into gas as the gun is fired. It is effected by the weather and the paintball gun itself has to be tuned more to compensate for fluctuations in climate conditions.
Written on the tanks is either 12oz or 20oz, this relates to the capacity and the pressure is the same at around 700PSI To fill the Co2 tanks requires a fill station which attaches to a master Co2 tank which one can hire from any gas supply like BOC gas's. Co2