"Special Sets", more commonly known as, "Armor Sets" are full sets of unique gear that provide various different attributes to the user. Armor sets can be both heroic and diamond.
Heroic Armor
Heroic Armor is a special type of armor that is stronger than diamond. Heroic Armor can be both specialty and non-specialty. Diamond Armor, both specialty and non-specialty, can be upgraded to it's heroic counterpart using a Heroic Upgrade.