Airsoft is divided into two catagories.
1. Mil-sim, or military simulation, is when a large group of people buy realistic military gear (Eg. Camo, vests, guns) and play large scale objective games, some of which can last for multiple days. Airsoft guns use a variety of propellants, Green Gas, electrically powered springs or manually cocked springs to propell a BB out of the barrell. It is comparable to paintball, however paintball markers shoot faster and harder than airsoft guns. The point of mil-sim games change, but basically it requires one team to fulfill an objective and the other to stop them. Transportation of certain objects, such as bombs and detanators, or the rescue of a survivor behind enemy lines may be used as objectives. These games are usually played on large fields with official refferees and don't end until one team acheives and objective.
2. Recreation: Recreational airsoft can be played (most likely not legally) in someone's back yard. Low grade guns are usually purchased, and there are no legal ref's or rules, and it usually works on the elimination system.
As a whole, Airsoft works on the honor system, as it is hard to see if you hit someone unless you are very close to them. It also has a very good binding element, if you're working with someone then you're bound to talk, and even post-game chatter with people on the other team can be constructive. Generally talking like an asshole is not accepted, and saying you pwned
someone is not taken well.