Certain talents and powers provide bonuses for heavy armour. If you look at an item, it’ll tell you whether or not it’s classified as heavy armour, but the following is a complete list:
Head : Full helm (also protects face and neck, but blocks spiked collar). Neck : Spiked collar, braided collar (epic) and torc of eternal health (epic). Body : Breastplate (also protects the back). Groin : Codpiece. Arms : Bracers, battle bracers (also protects hands) and runic bracers (epic). Hands : Gauntlets (not usable by Demon Form, so use battle bracers instead). Legs : Greaves. Feet : Boots (not usable by Demon Form). Wings : Wing-spikes and barbed wing-spikes (epic). Not for human form.
Dragons instead have a champron on their head, a criniere on their neck, a peytral on their chest, flanchards or reinforced flanchards on their flanks and a croupiere on their hind quarters. Their dragonspurs are also treated as heavy armour, as are their wing-spikes (as listed above).
Draconians wear the same as humans, except they must wear a champron instead of a full helm, and they can also wear dragonspurs and wing-spikes.