The sport of airsoft is a fun hobby for people that are looking for a real world war simulation. There are many things that separate the sport of airsoft from paint ball. The primary difference is the equipment that is used. In paint ball, you have non replica gun that shoots large balls of paint. The primary design and function of these weapons is around delivering the paint on target as effectively as possible. The goal is not to simulate a real rifle, or even a facsimile of one, but rather to fire paint as quickly and accurately as possible. In airsoft, the guns are designed to look and feel like the real thing. The primary goal in airsoft is in the actual simulation of combat. The guns will not fire like the real thing, but do weigh and look very realistic.
The other main difference between the two games is the rules. When a player in paint ball is hit it is very obvious. The rules of the game state that the paint ball has to break and be on the persons clothing for it to be considered a hit. Referees in the game judge whether or not a person is out. In airsoft the game is played more on the honor system. Since most rounds fired from an airsoft do not leave a visible mark on the player it is up to them to call themselves out. In some games there are referees to help with this as it is not always obvious to a player that they have been hit. If they are wearing many layers of clothing it can be difficult to detect the impact of the round. Some games will use rounds that are covered in paint and leave a mark, although these games are not nearly as common. While on the surface paint ball and airsoft may seem to be very similar, in fact they are quite different as illustrated here http://www.defconairsoft.co.uk/airsoft-vs-paintball/.