Automatic Electric Gun (A.E.G.) - These guns are powered by batter packs which powers an electric motor. The motor powers a series of gears, which wind and compress a spring connected to a piston and cylinder mechanism. When the spring is completely compressed, it is released and exerts force on the piston and cylinder mechanism. This forces compressed air through a nozzle and onto a BB, which is propelled out of the gun. Stock AEGs can deliver BB speeds of 240fps to 310fps using a standard 0.2g BB. Upgrading the spring and the supporting gears can easily deliver BB speeds of over 400fps or more.

Gas Gun (GBB and non-blowback guns)- Another popular type of an Airsoft gun is the gas-powered. While there are several types of gas-powered airsoft guns, the most popular are the Gas-Blowback guns (GBBs). The GBB allows a realistic, semi-automatic firing of the gun by employing either an on-board or magazine-stored gas storage system. The same compressed air that propels the BB out of the bore is harnessed to cycle the slide back (hence: blowback). This creates the realistic 'recoil' by cycling the slide/upper receiver back-and-forth while firing the gun. This system gives the hand-held gun the capability to empty a magazine as fast as you can pull the trigger and just as easily reload the gun by inserting a fresh magazine. The non-blowback guns do not cycle the slide while firing the gun. Sometimes non-blowback gas guns are less powerful than the GBB.

Spring Gun (Springer)- With Spring-cocking guns you cock the spring first, and then fire. Cock spring, fire, cock, fire, cock, fire, etc. These guns are usually magazine fed and semi-automatic, but they must be hand-cocked after every shot. Most of the spring guns are weak in power. There are also some high power spring cocking sniper rifles such as the British Army SA80 and the US M16.Airsoft is paintball-type recreational game that is played in indoors (CQB), outdoors and at organized mass events that range from Vietnam re-enactment to fictional Special Forces operations. The common point of all of these events is the use of airsoft guns and airsoft gun accessories. Airsoft started as a hobby in Japan back in 1980s and is now common in many other countries.

Unlike in paintball BB hit from airsoft gun doesn’t leave a mark, but stings a bit. This allows use of smaller and more lightweight pellets that hurt less and make less mess. This also allows completely different design of the airsoft gun when compared with paintball guns. Many of our Japanese made airsoft guns are highly detailed products that have incredible attention to the detail. You will be surprised about how well some of these airsoft guns are constructed.
Airsoft guns range from affordable light powered 10-year old toy guns to serious hobbyist custom guns that cost several thousand dollars a piece. They are either spring, gas or electric operated and have special features such as various blowback mechanisms or accuracy barrels.
In most countries owning airsoft gun doesn’t require license or special arrangements.

Tokyo Marui Automatic Electric Guns explained
The Marui Automatic Electric Guns (AEG) are operated by central mechanism called Gearbox and electric motor that derivates from RC-cars. The electric motor uses small 8.4v battery as power source and rotates the gears inside the gearbox that in turn cock the piston back and release it forward in high repeating speed. This creates high rate of fire of about 750rds per minute with muzzle velocity of about 80m/s with 0.20g BB (264 FPS).
There are some individual differences in Tokyo Marui AEG electric gun series that are explained here:

AK-series: AK47/47S/B-Spetsnaz – Strong Version III gearbox, high accuracy and long range, highly upgradeable thanks to Version III gearbox. Considered as one of the more beautiful Tokyo Marui AEGs.
FA-MAS series: FA-MAS 5.56 F-1 / FA-MAS SV – Version I gearbox, extremely high rate of fire. First Tokyo Marui AEG.
G3 series: G3 SAS / G3-SG1 / MC51 – Version II gearbox. Both G3 SAS and MC51 are popular with airsoft players who go for maximum gaming performance. G3 SG-1 provides platform for light/medium airsoft sniper rifle.
M4 and M16 series: M4A1 Carbine / M4 RIS / M16A1 / M16A2 / Knight’s SR-16 M4 – Version II gearbox. Have the highest number of accessories and custom parts available due to this series popularity among airsoft players.
MP5 series: MP5K / MP5K PDW / MP5 A4 / MP5 A5 / MP5 SD5 / MP5 SD6 / MP5 RAS / MP5-J - MP5K and MP5K PDW have Version III gearbox while others have Version II gearbox. Version III is stronger and better suited for upgrade. All of these Tokyo Marui MP5 AEGs are highly reliable, accurate and affordable. MP5 RAS and MP5-J are the latest versions and have number of gimmicks and new features.