Airsoft guns have three (3) types of power-plant options [systems for propelling projectiles (BBs)] and they are spring power, gas power and electric power.
Spring Power - Spring Airsoft guns must be cocked before each shot. These guns are among the least expensive Airsoft power-plants. An internal spring is compressed when the gun is cocked and a squeeze of the trigger releases the spring to propel an internal piston. The force of the spring propelled piston generates air pressure in the chamber of the gun behind the BB. The BB acts as a pressure release valve to exhaust the chamber pressure. The chamber pressure pushes the BB down the barrel, out of the muzzle and towards the intended target. Voilá, the functioning of the springer power-plant. Spingers can be among the most accurate and reliable of the Airsoft family of power-plants. This is mainly due to the fact that fewer and simpler mechanical parts are required for springers. Another benefit of springers is that the power source is self-generated each time the gun is cocked. There are no batteries to charge nor gas reservoirs to refill. This is a practical choice for individuals who are interested in entry into Airsoft guns.
Gas Power - Gas Airsoft are usually associated with pistols due to the insufficient room to store high-powered electric gear propulsion mechanisms. There are efficient gas powered rifles too. A feature included with many gas powered Airsoft is the ability to adjust velocity. This can come in handy when you're adjusting your gear to qualify for specific fps requirements. Most Airsoft gamers will carry a gas powered pistol as a side arm. Gas power is generally associated with guns that have "blowback" features. Blowback is when the propellant (gas) is used not only to send the BB on its way, but also to operate the slide of a gun to recreate the realistic firing cycle of a semi or fully automatic weapon. The common gases are Green Gas and HFC-134a. The less common options are CO2, nitrogen and red gas. Refer to your owner's manual to determine which gas options are suitable for your gun.
Electric Power - Automatic Electric Guns (AEG) are among the most popularly used Airsoft in the industry. The arrival of AEGs on the Airsoft scene caused a cataclysmic explosion in the Airsoft industry. AEGs are outfitted with an onboard rechargeable battery, which powers an electrically operated mechanical gear box (mechbox); AEGs provide gamers with a formidable weapon of realism and battle functionality. The mechbox compresses the mainspring and releases it to create the chamber energy that discharges each shot when the trigger is squeezed. During the decompression cycle, the mainspring propels a piston that results in a sudden air pressure behind the BB. Whether in semi or full-auto mode, the mechbox cycles shot after reliable shot. The AEG is the preferred power-plant of die-hard skirmishers.