This list is compiled mostly from the powerwiki over at aiononline.com.
I have changed some of the wording to make more sense when things, I felt, needed better clarification.

I thought it might be helpful to others to have all this information in one place.

So ... without further ado ...

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Hit Points (HP) are the numerical value representing the amount of damage a player or NPC can take before their demise.

Mana Points (MP) are the numerical value representing the amount of magical power a player has available.
If not enough points are available skills and spells will not activate.

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CLASS ATTRIBUTES

There are six basic class attributes. These can be modified through the use of your own or another's skills.

POWER defines a character's basic physical strength and affects physical damage inflicted on enemies during combat.

HEALTH defines a character's basic hit points and hit point recovery.

AGILITY defines a character's basic ability to evade, parry or block enemy attacks.

ACCURACY defines a character's basic to hit and chance to critically hit an opponent while in combat.

KNOWLEDGE affects spell damage and spell accuracy.

WILL defines a character's basic mana points and mana point recovery.

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PHYSICAL DEFENSE is a character's ability to mitigate melee and ranged physical damage.
The higher this value the less damage taken when hit with physical attacks.

EVASION is your character's ability to dodge attacks.
Evading an attack is the only way your character can completely avoid damage from a physical attack.

Note: Spells and ranged attacks can sometimes miss you, however,
that is based on the character or NPC's MAGIC ACCURACY rather than your Evasion.

BLOCK is your character's ability to block and reduce damage from attacks.
Blocking an attack requires that you have a shield equipped.
Depending on the attributes of the shield, you will see various levels of damage reduction.

PARRY is your character's ability to defend itself with weapons.
When a parry is made your character will still take damage however it will be at a reduced percentage.

MAGICAL RESISTANCE is your ability to resist incoming spells.
This attribute gives you the chance to avoid damage spells and debuff spells.
This is also the only attribute that affects spells in this way.

FIRE, WIND, WATER and EARTH RESISTANCE lowers the damage taken when struck by elemental based spells.
The higher this attribute is the less damage you will take.

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WEAPON ATTRIBUTES

ATTACK is an attribute that modifies physical damage done by weapons and abilities.

ACCURACY again; however, this time showing your accuracy with the current equipped weapon.

CRIT RATE is your chance to critically hit an opponent with a weapon. A critical hit results in higher damage.

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ATTACK SPEED is the delay between each normal melee attack made by your character.

SPEED is the rate at which your character moves throughout the world while running.

MAGIC BOOST is the modifier that increases spell damage.

MAGIC ACCURACY is your characters ability to hit a target.
The higher this attribute the more often you will hit what you are casting spells at during combat.