Airsoft sniper guns are typically the most powerful of all the airsoft guns, simply because they are intended for long range target practice.
Airsoft sniper guns shoot out the same plastic bb pellets that all airsoft guns shoot out, but the spring loader in the gun is much larger and more powerful than the pistol or even airsoft shotgun. Aside from the larger spring, there are also airsoft snipers that run on co2 to give an even more powerful shot. While the airsoft sniper is made to be more powerful than the other airsoft guns, the price of the sniper runs in the same direct correlation as all guns in quality. The higher the price for the gun, the higher quality of an airsoft sniper you will get. Being the most powerful of the airsoft class of guns, the airsoft sniper runs in the hundreds of dollars for the mid to upper class version of the firearm, and in the lower hundred dollar range for the entry level sniper.
The lowest quality of an airsoft sniper is typically equivalent to the mid to upper quality of a pistol in terms of power, and a mid range for the airsoft shotgun power range.
The airsoft sniper is best utilized for team play in a covered area. If you are buying the gun to play a capture the flag based type of airsoft game, then the gun is great for long range playing fields. The gun however is not the weapon of choice for smaller playing fields, but it still can be utilized for its power. Aside from the actual airsoft games, the sniper is great for target practice and introducing young ones to gun safety at a much more manageable level.
Overall if you play airsoft, or are interested in the guns, check out how an airsoft sniper can benefit you.