Its similar but much better than paintball. Essentially its a contact honor sport. You use replica guns and shoot plastic balls usually weighing 0.20 or 0.25 grams. Its pretty much like a COD game but a sport in a sense. If your hit you yell 'HIT HIT HIT' and proceed to the re spawn etc depending on the game mode. It hurts a little and can bruise, but it generally fades after a small amount of time pain wise. Can leave some sick looking bruises. Usually theirs different types of sites eg CQB - Close Quarter combat, and normal games. Generally CQB sites have engagement rules to prevent overkill, as being shot from a rifle that can shoot a plastic pellet at 300 feet per second at 1 meter away can sting a bit. You need mandatory eye protection, and lower face is very much recommended if you like your teeth. Usually its no full auto at under 10 meters.
Paintballing is gernally more well known but its messy, very expensive to play with the balls, whilst airsoft has the initial expense for the gun and gear. Its getting more and more popular worldwide with some places even having 2000 people play. With tanks ect being used as well. Crazy stuff.