Rifling is the lands and groves inside of the barrel. It causes the projectile to spin, therefore making it more accurate.
Smoothbore as no rifling whatsoever. It is smooth throughout the barrel.
Just because a gun has rifle sights, does not mean it has a rifled barrel. Normally, a shotgun only has a small bead at the very end of the barrel to act as it's sight. Some people prefer a rifle sight over this bead. It also allows them to shoot slugs out of their smoothbore barrel more accurately.
If the barrel is rifled, it is meant only for slugs. Buckshot or birdshot could mess up the rifling, and it will also throw bad patterns. (basically makes the patterns look similar to a donut)