The rarity ladder
The six named tiers are Common, Rare, Epic, Legendary, Mythic, and Ascended. With hundreds of items in the game, the label is useful for sorting loot, but it does not by itself describe every item's role in a build. Elemental affinity can also affect how a piece fits with your current equipment.
What duplicates can do
Duplicate items have a use in the forge: they can be put toward a chance at an upgrade. The available public description does not state a complete recipe, success-rate table, or rarity-by-rarity cost. Check the current in-game forge before spending items you want to keep.
A forged Legendary is account-bound under the published trading rules. That is a meaningful difference from a Legendary item dropped in battle.
Loot source matters
Four PvP bosses have gear that cannot be obtained elsewhere. Their complete item list is not published in the available first-party description, so this guide does not invent one.
If selling is part of your plan, record both an item's rarity and whether it came from battle or the forge. Those two details affect its eligibility for the Steam Market.