There are various items to be found in the world of Dungeon Defenders. The quality of an item generally determines how good the item will be. The higher quality items have lower drop rates than the lower quality items. The armor types grow stronger the further down the table they are (below).
With the introduction of Super Loot, any armor class such as Leather, Mail, Chain, Plate and Pristine has the potential of having better stats than normal thus making all armor as equal to each other.
The quality associated with an armor type determines the max stats it can attain. The better the quality, the better the max stats can be. It is possible to have a lower quality item that is better than the next level quality due to overlapping. Quality is the first adjective in an armor's name.
Level Requirement Chart
An armor's magical property is another indication of approximately how good an item is. There are currently 5 levels of magic that an item may be. From worst to best, these are Cursed, Unlucky, Lucky, Blessed, Enchanted. Generally, Cursed items have at least one negative stat and often times more than one. As with armor Quality, the magical property is not an absolute indication of item value/usefulness. Magical property is the second adjective in an armor's name.
It may be possible for an item to NOT have a magical property. 
The different types of armors have certain restrictions as to how high stats will drop on them. Leather will usually have lower stats than pristine of the same quality. Leather armor can increase resistances as much as plate or pristine. Armor type is the second to last word of an armor's name, while the last is always the slot of the item.
Resist allows you to absorb or negate damage taken by mobs or players in PvP, thus reducing the total damage taken. Resist come in four types:
Reduces all Physical Damage from mobs, including the Ogre and Dark Elf Warrior.
Reduces all Fire Damage from mobs, including the Wyvern, Dark Elf Mage, Kobolds and Ancient Dragon.
Reduces all Lightning Damage from mobs.
Reduces all Poison Damage from mobs including the Ogre's Poison Ball.
A guide is available for more information: Armor Resistance Guide
(PC) When you equip armor of all the same type in every slot, every piece gains a bonus in all stats on the armor. You will know this is in effect when you see a pulsing cyan square around the armor icon's border. Pets and weapons do not gain this bonus. All armor gains 25% boost on all stats. (The math for the bonus is 1+ an extra bonus point every 4 levels starting at level 5. So at 1=2, 5=7, 8=10, 9=12, etc.)
(mobile?) Some armor wield Set bonuses for having a specific amount of that armor type. set bonuses include resistance, extra damage resistance, and other stats such as increased tower HP and attack.
First it is worth looking at the strength of the items (i.e. torn, worn, sturdy, etc.) to get a basic idea of how upgrade-able it is, and how long you will probably want to keep it. Upgrading is not a cheap process, so spend wisely. This number will state how many times the item can be upgraded. There are two methods to upgrade items:
Method 1: 1. Go into the lockbox 2. Click on Hero Info or X 3. Click on an item 4. click A 5. Hold down the right trigger to invest in it (cost is shown on lower right side), and press A to confirm
Method 2: 1. Store items to be upgraded in your lockbox (sell all of your junk items to avoid clutter) 2. Talk to the Tavernkeep 3. Click on Items, on the left side of the menu 4. Choose the item you wish to upgrade 5. Hold down the right trigger to upgrade (cost is shown on lower right side), and press A to confirm
Many items can have multiple levels to be upgraded (shown at the bottom of the item picture), so you can keep going till they are all full. It is also important to note that all of your characters share the lockbox items and money (mana).