Uniques and Set items have preset mods, though the values of these mods may vary somewhat. A given item might spawn with somewhere between +100-150% damage, for instance. Uniques are golden named items, and there is just one unique per item type, in most cases. Uniques are usually very good items, but are seldom the best possible item of that type. They often usually special items though, with mods not possible to find on magical or rare items.