The definition of balance is the uniform distribution of mass about an axis of rotation, where the center of gravity is in the same location as the center of rotation. In English, that would translate to...A balanced tire or is when the mass of the tire, when mounted on its wheel and the car's axle, is uniformly distributed around the axle. Even easier yet, how about...there are no heavy spots.
Balanced tires can be the difference between a good or bad driving experience. Some cars (and drivers) are more sensitive to an out of balance tire than others, but no one is happy with a vibration.
An out of balance tire can adversely affect ride quality, shorten the life of your tires, bearings, shocks and other suspension components. If you have a vibration that is dependent on speed, and usually becomes noticeable around 40-45 mph and increases as your speed increases, it's probably balance related. The other primary cause of vibrations is that the tire and wheel assembly isn't perfectly round. Face it, if we go out far enough past the decimal point, nothing is perfectly round. This includes your wheels and tires. The problem is when the high spot on the tire, and the high spot on the wheel end up being matched to each other. This effectively doubles the amount of "hop" or runout. If re-balancing doesn't cure the vibration problem, have your professional installer check the runout of the tire. If there is a "hop", many times the problem can be fixed by simply rotating the tire on the wheel slightly. The technician should loosen the tire on the wheel, and turn it 180 degrees, and reinflate the tire after relubricating the bead. The runout should be significantly reduced or eliminated, and if it's not, try it again, but this time rotate only 90 degrees, and if that doesn't work, try 180 degrees on the third try. Done this way, the high spot on the tire has been tried at each quarter of the wheel. At one of those points, the tire should be good and round. At that point, rebalance the tire, and go for a test drive. If the vibration persists, the problem is either in the tire, or elsewhere in the vehicle.