Damage type

Any damage done has a type. Multiple types of damage can come from the same source (e.g. a flaming spear would do both fire and piercing damage). It is possible to protect from each type.

Physical
All weapons have a physical damage type. Some spells also cause physical damage. Damage reduction can reduce the amount of physical damage done.
 * Slashing: axe blades, sword-edges
 * Piercing: spears, arrows, spikes
 * Bludgeoning: clubs, hammers

Energy
This is usually in addition to a weapon's base damage, or from a spell effect.
 * Fire
 * Cold
 * Electricity
 * Sonic
 * Acid

Others

 * Magic: pure magic energy
 * Sacred: the power of divine "good"
 * Profane: the power of unholy "evil"