Character Stats

HP (Hit Points)
Your health. When it reaches 0 (Or in rare cases, -1), you die.
When you have only 20% of your health and less, your character starts to blink red and you should drink some health potions or nexus before you take any further damage.

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 if Weak = 0.5
Damage Multiplier normal = 0.5 + ATT / 50 = (ATT + 25) / 50
Damage Multiplier with Damaging = Normal * 1.25
Therefore,
Damage = Random(MinDamage, MaxDamage) * Damage Multiplier
Starting with a base of 50% of weapon damage at 0 ATT, each point increases damage by 2% of weapon damage. Examples:
10 ATT Priest with a Fire Wand (30-55 DMG) = 21-38 DMG
83 ATT Warrior with a Sword of Acclaim (220-275 DMG) = 475-594 DMG
Note: In game, the highest weapon base damage value is actually never rolled. Thus the effective weapon base damage range always is one less than stated in the weapon description. For example, the effective weapon base damage range of the Fire Wand (originally 30-55 DMG) is 30-54 DMG. Taking this into account, the previous two examples would change as follows:
10 ATT Priest with a Fire Wand (30-54 DMG) = 21-37 DMG
83 ATT Warrior with a Sword of Acclaim (220-274 DMG) = 475-591 DMG

DEF (Defense)
Decreases the amount of damage taken.
Straight 1 point per 1 damage reduction, but caps at 90% of total damage. This means that every shot will do at least 10% of its damage to the player. Example:
60 DMG Attack against 20 DEF = 40 DMG
60 DMG Attack against 54 or higher DEF = 6 DMG

SPD (Speed)
Increases the speed at which the character moves. Base (0) is 4 tiles per second, then each point adds about 0.07467 tiles per second.
Actual speed in T/s (tiles per second) = 4 + 5.6 * (SPD / 75) = 5.6 * (SPD + 53.5) / 75
50 SPD: 7.733 T/s
75 SPD: 9.6 T/s
T/s while Speedy: Normal * 1.5 (75 SPD with Speedy means 14.4 T/s movement speed)

DEX (Dexterity)
Increases the speed at which the character attacks. Base (0) is 1.5 attacks per second, then each point adds about 0.0867 attacks per second.
A/s (attacks per second) = 1.5 + 6.5 * (DEX / 75) = 6.5 * (DEX + 17.3) / 75
50 DEX: 5.833 A/s (350 per minute)
75 DEX: 8 A/s (480 per minute)
A/s while Berserk: Normal * 1.25 

VIT (Vitality)
Increases the speed at which hit points are recovered. Base (0) is 1.0 HP per second, then each point adds about 0.24 HP per second.
HP/s (hp per second) = 1 + 0.24 * VIT = 0.24 * (VIT + 8.3)
40 VIT: 10.6 HP/s
75 VIT: 19 HP/s
HP/s while Healing: Normal + 20 (75 VIT with Healing means 39 HP/s)

WIS (Wisdom)
Increases the speed at which magic points are recovered. Base (0) is 0.5 MP per second, then each point adds about 0.12 MP per second.
MP/s (mp per second) = 0.5 + 0.12 * WIS = 0.12 * (WIS + 8.3)
50 WIS: 6.5 MP/s
75 WIS: 9.5 MP/s
Some abilities gain an increase in power if Wisdom exceeds 50 (30 for Warriors); this is called WisMod. A high Wisdom stat increases the intensity, range, and duration of Priest Tomes, Paladin Seals, and Mystic Orbs, the radius and damage of Necromancer Skulls, the number of targets and damage of Sorcerer Scepters, and the radius and duration of Warrior Helms. Some attributes improve with every point of Wisdom, typically damage and radius. Other attributes only improve every ten points of Wisdom over the threshold, such as the duration of applied status effects. Certain untiered items may have smaller or larger increments of Wisdom required to improve an attribute.

For example: A Priest with 75 Wisdom using a tiered tome will see an improvement of 2.5 tiles to their tome’s radius, 2 seconds to the duration of the applied healing status effect, and an additional 60hp healed. If the Priest equips an item to improve their Wisdom to 80, the radius will increase to 3 tiles, the healing duration to 3 seconds, and their tome will heal an additional 90hp.

See also: