Typically however many weapon damage dice rolls + the listed ability modifier + any feat bonuses+ any enhanced item bonuses + any miscellaneous damage modifiers (such as a versatile weapon being used as a two-handed weapon.
Damage types[]
Many powers create energy or a substance that deals damage to their targets. Types of damage often have consistent secondary status effects that accompany them. Certain creatures also have favored types of damage to inflict (Black Dragons deal primarily acid damage, Demons deal mainly necrotic damage, etc.). Not all attacks have typed damage.
Acid: Corrosive liquid.
Cold: Ice crystals, arctic air, or frigid liquid.
Fire: Explosive bursts, fiery rays, or simple ignition.
Force: Invisible energy formed into incredibly hard, yet nonsolid shapes.
Lightning: Electrical energy.
Necrotic: Purple-black energy that deadens flesh and wounds the soul.
Poison: Toxins that reduce a creature’s hit points.
Psychic: Effects that target the mind.
Radiant: Searing white light or shimmering colors.
Thunder: Shock waves and deafening sounds.
Varies : If the notation “Varies” appears among a power’s keywords, that means the power has variable damage types, which the user or the circumstances determine.
Adding and Removing Damage Types: If a power gains or loses damage types, the power gains the keywords for any damage types that are added, and it loses the keywords for any damage types that are removed. The poison keyword, however, is removed from a power only if that power neither deals poison damage nor has any nondamaging effects. [Rules Compendium:115]
Example: If a characther casts a lightning power through a magic staff that changes the damage to fire, the power gains the fire keyword and loses the lightning keyword for that use, since the power is dealing fire damage instead of lightning damage. That use of the power can therefore benefit from game elements, such as feats, that affect fire powers, but not from those that affect lightning powers.
Extra damage[]
Many powers and other effects grant the ability to deal extra damage. Extra damage is always in addition to other damage and is of the same type or types as that damage, unless otherwise noted. Because of this rule, an effect that deals no damage cannot deal extra damage. However, a power doesn’t necessarily have to hit a target to deal extra damage—it needs only to deal damage to the target.[RC:223]
Example: A cleric might have an ability that causes her to deal 5 extra radiant damage to undead creatures. That ability means she deals 5 extra radiant damage whenever she deals damage to an undead creature, and the damage is added to the original damage.