Primary Stats
STR
Strength: Every 1 STR adds 0.3 Melee Damage. Every 5 STR adds 1 Critical Additional Damage

DEX
Dexterity: Every 1 DEX adds 0.3 Accuracy, 0.3 Evasion. Critical Chance is reflected by class.

INT
Intelligence: Every 1 INT adds 0.3 Spell Damage. Every 5 INT adds 1 Skill Critical Additional Damage.

CON
Health: Every 1 CON adds 5 HP. Every 5 CON adds 1 to HP Recovery

MND
Spirit: Every 1 MND adds 4 MP. Every 1 MND adds 0.3 HP Restoration to direct heal spells. Every 5 MND adds 1 to MP Recovery. Skill Critical Chance is reflected by class.

Substats
HP & MP Recovery
HP and MP Recovery occurs once every 3 seconds. The amount regenerated depends on the state of the character, as well as equipment, and stats.

Sitting
A sitting or resting character gains 100% of their HP and MP Regen.
Standing
A standing character gains 50% of their HP and MP Regen
Combat
A character in combat gains 30% of their MP Regen.
As of 6/17/08 Heimdallr posted on official Requiem Forums that "HP does not regenerate naturaly during combat." Source

Defense
Every 10 Defense reduces Melee Damage by 1 point.

Block Chance & Block Defense
Block Chance is the percent chance that a melee attack will be blocked. On a successful block, Block Defense is the amount of damage that that melee attack is reduced by.