A long gun is one with an extended barrel and commonly with a shoulder butt. Barrels of such weapons commonly extend to around 30 inches and this gives additional accuracy and range. Most long guns fall into 2 categories, rifles and shotguns (also refered too as smooth bore).
The rifling of a rifle barrel is a series of grooves within the interior of the barrel itself which cause the projectile to spin whilst in flight, increasing the stability of the projectile and thus the accuracy and range of the weapon.
A shotgun on the other hand is a short range weapon which uses its longer barrel to give increased power to the pellets that are shot out. These long guns use a smooth bore as the pellets tend to jam if there is any kind of obstruction in the barrel and due to the short range and spread of the weapon it is not needed.