The best visual example of an overpronated foot is what is normally called a flat foot. This may in itself not be painful. An overpronated foot places more pressure on the inside of the foot, causing incorrect joint positioning in the knee, hip and opposite shoulder. Overpronation also causes an imbalance of the muscles in both the upper and lower leg. The inner leg muscles are underdeveloped and used while the outer leg muscles are overused and abused. This disturbs the core balance of the body.
underpronation: the opposite of flat feet, a high arch
seriously, this explains it pretty well: http://www.time-to-run.com/beginners/shoes.htm
These are the first sites (literally) that google comes up with, so I'm sorry if you already read them and they're the ones you found confusing.
Regardless, if you're serious about running you should go to a good running store and have them explain everything to you (good = testing and explaining).
And: http://www.geocities.com/jsgilbody/Gord ... 040104.pdf