This guide is intended to help with understanding the many flags, bonuses and attributes that can be found on items and equipment.
These terms are found on equipment and other items in the game to describe their quality or rarity.
COMMON - Common items do not actually have a "common" flag on them, but instead are items that have none of the other quality descriptors listed below. They are of very low quality and are generally considered "trash loot" (loot to sell to vendors).
UNCOMMON - Uncommon items are dropped in small chests or body loots on monsters. They are of very low quality and are generally considered "trash loot".
HANDCRAFTED - Handcrafted items are made by tradeskillers; these items are usually lower in quality then their treasured counterparts and use common harvested materials to create.
TREASURED - Treasured items are dropped in chests by monsters and are commonly given out as quest rewards. For equipment items typically are a bridge in quality between Handcrafted and Mastercrafted. Many pieces of equipment and spell scrolls that are treasured quality or higher can be transmuted.
MASTERCRAFTED - Mastercrafted items are made by tradeskillers; these items are superior in quality compared to Handcrafted and Treasured items but are somewhat lacking compared to Legendary.
MASTERCRAFTED-LEGENDARY - Mastercrafted-Legendary items are made by tradeskillers; these items are a stepping stone in quality between Mastercrafted and Legendary items.
LEGENDARY - Legendary items are dropped in chests by monsters, purchased from faction or token merchants, are rarely created via tradeskill combines or are sometimes given out as quest rewards (typically from Heritage or Heroic quests). The monsters that drop these items are usually Heroic encounters. Legendary equipment is an upgrade in quality from Mastercrafted but not quite as powerful as Fabled.
MASTERCRAFTED-FABLED - Mastercrafted-Fabled items are made by tradeskillers; these items are a stepping stone in quality between Legendary and Fabled items. Prior to Destiny of Velious there are no known Mastercrafted-Fabled items in game.
FABLED - Fabled items are dropped in chests by monsters, purchased from faction or token merchants, are rarely created via tradeskill combines or are rarely given out as quest rewards (typically from Epic quests). The monsters that drop these items are usually Epic encounters. Fabled equipment is considered to be one of the highest qualities in the game.
MYTHICAL - Mythical items are currently only found as rewards for completing the mythical version of a classes' epic weapon quest, as well as a couple of epic-encounter dropped artifact items. This is the highest quality in the game.
These flags are often found on items to allow or prevent flexibility with how the items works, how certain game components can interact with it or how the item can be passed between players.
APPEARANCE-ONLY - Equipment with the appearance-only flag can only be worn in the appearance tab of the character window.
ARTIFACT - An artifact is an item of which only one of its kind typically can exist on a server.
ATTUNABLE - This item must be attuned to the character that wishes to wear it in order to receive its benefits. When attempting to equip an attunable piece of equipment a confirmation box will confirm your decision to attune to that character. Once confirmed, this action is irreversible.
ATTUNED - A piece of attuneable equipment that is already linked to the specific character that is wearing it. Generally speaking items cannot be unattuned, the only current exception being with a rare Legends of Norrath loot item.
HEIRLOOM - These items can be traded but only through your shared bank with other characters on the same account. The heirloom flag makes the items essentially no-trade to other players. With Destiny of Velious these remain tradable for 48 hours with any character who was in the group when the encounter was defeated. Once the item is attuned or adorned, it will no longer be tradable with those characters.
LORE - The lore flag means that you can only carry one of each lore flagged item on your character. The exception is that multiple of the same items with a lore tag can be placed in shared bank or a guild bank.
LORE-EQUIP - An item with lore-equip means that only one of a given item with that flag can be equipped, yet duplicates can still be carried in the inventory. Items with this flag are generally of types that can be equipped in pairs, such as rings, weapons and earrings.
NO-DESTROY - Items with a no-destroy flag cannot be destroyed by the player. Generally these items are needed for quests.
NO-TRADE - Items that are no-trade cannot be given to any other character, nor can be placed in the shared or guild banks. With Destiny of Velious these remain tradable for 48 hours with any character who was in the group when the encounter was defeated. Once the item is attuned or adorned, it will no longer be tradable with those characters.
NO-VALUE - Items that are no-value have no sell value to a merchant. If they are tradeable then they can still be sold on the broker. No-value items cannot be transmuted.
NO-ZONE - Items that are no-zone will disappear from your inventory upon leaving the zone it was given in. These items are typically quest items.
ORNATE - Ornate items cannot be adorned or transmuted.
TEMPORARY - Temporary items will disappear from your character a short time after camping out.
UNLOCKED - The unlocked flag previously only existed on Extended servers; this flag is placed on gear that is normally too powerful for a subscription level to be permitted to equip (Legendary through Fabled quality) when the player uses a Marketplace purchased gear unlocker on it. The item will then retain this flag indefinitely.
Attributes are also known as statistics (stats) and are generally the green colored bonuses from items. For Agility, Intelligence, Stamina, Strength and Wisdom, once 1200 is reached, the progression becomes: a 30% stat gain = a 10% boost to what the stat provides. 
Crushing - How well you perform with the crushing skill in melee combat if you have a crushing weapon (such as a mace or staff) equipped.
Defense - How well you avoid physical attacks.
Health - The total amount of health. This is increased through stamina and is depleted by taking damage in combat, from falling, from detrimental effects or by failing to counter tradeskill events. In general, once your health falls to 0 you will die, though some class-specific alternate advancements will allow a player to fall below 0 health. With the release of Destiny of Velious, the number of hit points you gain per level was increased. Hovering over your stamina on the character window should tell you the per point bonus based on your level.
Mitigation - How well you absorb the damage from melee attacks.
Physical - Enhances how well you mitigate melee attacks from your opponents. Sometimes considered to be a resist.
Piercing - How well you perform with the piercing skill in melee combat if you have a piercing weapon (such as a dagger) equipped.
Power - The total amount of power. This is increased through stats and items and is consumed while using abilities or tradeskill counters. With the release of Destiny of Velious, the number of power points you gain per level was increased. Hovering over your primary attribute on the character window should tell you the per point bonus based on your level.
Slashing - How well you perform with the slashing skill in melee combat if you have a slashing weapon (such as a sword or axe) equipped.
These attributes enhance how a tradeskiller performs while making a combine.
Critical Success Mod - Percentage based increase to your chance of critical success.
Durability Add - Percentage based additive to your durability gained.
Durability Mod - Percentage based increase to your durability loss or gain.
Progress Add - Percentage based additive to your progress gained.
Progress Mod - Percentage based increase to your progress gained.
Success Mod - Percentage based increase to your chance of success.
"Blue Stats" are item or alternate advancement modifiers to your adventuring abilities, spells or even your character.
Ability Modifier - This modifier will add a flat amount to the effectiveness of an ability or spell. This flat adder is capped at 50% of that ability's base. 
Accuracy - Percentage based accuracy of your melee weapon attacks.
Block - Raises your chance to avoid an attack (avoidance). This is a percentage based reduction.
Block Chance - Raises your chance to avoid an attack by blocking it. This is a percentage based reduction.
Casting Speed - Percentage based reduction to your ability casting speed.
Crit Bonus - Percentage based chance of how hard your spells and abilities will critically hit for. There are four separate categories for crit bonus: Melee, Spell, Heal and Taunt.
Crit Chance - Percentage based chance of how often spells and abilities will strike for critical damage.
Critical Mitigation - Reduces the damage of critical attacks in PvE combat. This is a percentage based reduction.
Damage Per Second (DPS) - Percentage based modifier that raises your auto attack damage.
Deflection - Raises your chance to avert a physical attack if you are a Brawler.
Haste - Percentage based modifier to how quickly you will attack with melee weapons.
Hate Mod - Percentage of extra hate that you will generate. Also known as Aggression or Hate Gain.
In-combat Health Regen - How much health you regenerate per update while in combat.
In-combat Power Regen - How much power you regenerate per update while in combat.
Melee AE - Percentage based chance to hit multiple targets from a successful auto attack. These types of attacks are normally non-discretionary. Base chance is 0%; usually the only way to raise it is by using a buff or alternate advancement ability.
Multi Attack - Percentage based chance to hit multiple times with a single melee attack. The higher the percentage, the more likely you will make more attacks. This is capped at 100% until the release of Destiny of Velious. Formerly known as Double Attack.
Parry - Raises your chance to ward off a physical attack if you are a class with the Parry skill.
Potency - This increases the base percent for any ability or spell you use. Potency is a percentage increase to the effectiveness of all of your abilities. 
Recovery Speed - Percentage based reduction to your ability recovery timers.
Reuse Speed - Percentage based reduction to your ability or spell reuse timers. Also known as Ability Reuse Speed or Spell Reuse Speed.
Riposte - Your chance at striking back at an enemy after they strike you in melee combat if you are a class with the Riposte skill.
Run Speed - This is the higher value of either foot or mount speed. Base run speed is 0%. Maximum run speed is 100%. Aside from character traits and alternate advancement abilities, some items stats, buffs and item effects can also raise foot or mount run speed.
Strikethrough - Percentage based chance of your chance to pierce through defenses.
Toughness - Percentage based chance to reduce PvP damage and/or critical damage.