Airsoft is a competitive team shooting sport in which participants shoot opponents with spherical plastic projectiles launched via replica air weapons called airsoft guns. Read more on Wikipedia.
Airsoft guns shoot 6 mm plastic BBs. They are replicas of a real firearms, both historical and modern ones. The power of airsoft guns varies - some replicas are weak and not dangerous, while some are very powerful and can penetrate a tin can. Eye protection is always a must while operating an airsoft gun!
In most of the countries airsoft replicas are not considered a weapon by law, so you don't need any permissions to buy or use one. However, to be sure, check your local regulations.
There are 3 types of airsoft guns:
Spring guns - need to be cocked before every shot. Very reliable, works in low temperatures, do not need gas or batteries.
Gas guns - uses CO2 cartridges or are filled with green gas. Semi or full auto, very powerful and realistic. Do not work in low temperatures - +5°C or more are recommended.
Electric guns - uses batteries and a motor to self reload. Semi and full auto. The cheaper models are light and weak, while the more expensive ones (more than 120 €) shoots very fast, precise and powerfully. Easy maintenance.