The three classes are durability, accuracy, and damage.
The following table lists each class and its levels, best to worst.
Durability note: the term indestructible is misleading. Like all weapons, indestructible weapons will, in fact, eventually destroy from use (although they are the most durable of the group).
Accuracy note: The only skill that actually becomes more accurate is archery. All other fighting skills get a bonus to their tactics, which in turn increases damage.
Damage note: Exceptional weapons crafted by smiths or fletchers have their own bonus and do more damage than weapons of ruin or might.