Damage
Damage
Upgrades and items have to main ways of enhancing your damage: base damage and power scaling (attack or skill power).
Calculating damage dealt (enemy resistance not considered) = (Base Damage)*(Attack Power*2%)
e.g. 5 base damage with 20 attack power = (5)(20*2%)= (5)*(1.4)= 7 damage dealth
Damage Types
Items that increase base damage (physical or magical), attack power, or mentions is primary exclusive only increase the strength of your primary attack.
Items that increase skill power, skill crit chance, skill crit multiplier increase the damage of every skill that is NOT a primary attack, it will even enhance the base damage of passives such as Paladin Juggernaut and Priest Celestial Orbs are affected.
Armor penetration and magic penetration is specific to the damage type of your skill. Note that hybrid damage type primaries and skills will only receive 50% effective benefit out of penetration items.