Character Stats Explained (Rev. 1)

HP (Hit Points) - Your health. When it reaches -1, you die.
When you get hit by a bullet that does the same amount of damage as you have HP, your health will stay at 0. You will live, but you should drink a health potion fast, as ANY enemy can kill you. Don't nexus, because that will kill your character.

MP (Magic/Mana Points) - Your magic. This is expended to use abilities. Usage depends on what class and ability item tier you are using.

ATT (Attack) - Increases the amount of damage done by weapons. Attack does not affect ability damage.
Damage Multiplier = 0.5 + (ATT / 50)
Therefore,
Total Damage= Base Damage * (0.5 + ATT / 50)
Starting with a base of 50%, each point increases 2%, examples:
10 ATT Priest with a Fire Wand (20-40 DMG) = 14-28 DMG
83 ATT Warrior with a Sword of Acclaim (220-275 DMG) = 475-594 DMG

DEF (Defense) - Decreases the amount of damage taken.
Straight 1 point per 1 damage reduction, but caps at 85% of total damage, example:
60 DMG Attack against 20 DEF = 40 DMG; 60 DMG Attack against 51 or higher DEF = 9 DMG.

SPD (Speed) - Increases the speed at which the character moves.
actual speed in TPS (tiles per second) = 4 + 5.6 * (SPD / 75)

DEX (Dexterity) - Increases the speed at which the character attacks.
APS (attacks per second) = 1.5 + 6.5 * (DEX / 75)

VIT (Vitality) - Increases the speed at which hit points are recovered. Base (0) is 1.0 HP per second, then each point adds about 0.12 HP per second.

WIS (Wisdom) - Increases the speed at which magic points are recovered. Base (0) is 0.5 MP per second, then each point adds about 0.06 MP per second.

See also:

Dex vs. Att
Def vs. HP vs. Vit
Class DPS