Lifted direct from Wikipedia

Airsoft is primarily a recreational activity with replica firearms that shoot plastic BBs that are often used for personal collection, gaming (similar to paintball), or professional training purposes (military simulations, a.k.a. MilSim, and police training exercises). A primary difference between airsoft guns and BB guns is that an airsoft gun uses a 6mm or 8mm plastic pellet and has a muzzle velocity of typically less than 180 meters per second (600 feet); which is generally considered safe when used in a controlled environment and with safety equipment like protective eyewear. A metal or alloy BB on the other hand, is projected at higher velocities (120-365 MPS) (400–1200 feet) or higher making them unsuitable for gaming and training purposes. Another distinguishing factor between BB guns and airsoft guns is that airsoft guns generally have some sort of “hop-up” mechanism that creates a back-spin on the BB to increase its accuracy and effective range. This hop-up feature is needed due to the light weight of the plastic pellet causing the plastic BB to otherwise fly uncontrollably, reducing its effective range.

Airsoft games vary greatly in style and composition depending on location, budget, and the quantity of participants but often range from short-term skirmishes and organized scenarios to military simulations and historical reenactments. Gaming with Combat situations on a mock battlefield mainly involve the use of common military tactics to achieve the objectives set in each game. Participants typically use varying types of airsoft weaponry along with either real or replica military gear and uniforms.