Handguns and rifles have rifled barrels, meaning that there are grooves cut lengthwise into the inside of the barrel. The grooves cause a bullet to spin, which makes it shoot out straighter and travel faster.
Most shotguns are not rifled inside. With standard ammo like lead or steel shot, a rifled barrel would cause the pieces of shot to bunch up into a tighter pattern, which would defeat the purpose of using a shotgun.
For shooters who to more tightly control the spread and impact point of their shot, there are chokes. These are tubes that use a cone or bumpy shape to taper the angle at which ammo leaves the barrel and the distance it travels. Some of them are rifled, and some are not. Some are even adjustable on the fly, meaning you can change the effect without removing the choke.
Choke manufacturers express their expected effects by listing the amount that a choke constricts the barrel and the percentage of shot that will hit a target area at 40 (or, in some cases, 25) yards. In general, the more the barrel is constricted, the higher the percentage of shot hitting the target at 40 yards. But this is all relative to the size and type of shot. Because of this and all of the variables involved (weather, wind conditions, individual barrel, etc.), it's not easy to say precisely how a particular choke will affect the shot pattern, and most shooters have to learn by trial and error.