Spell power

Spell power is an attribute that increases the effect of spells. Spell power increases the damage amount of damaging spells and the healing amount of healing spells.

Your bonus damage and bonus healing from spell power do not usually apply directly to a spell. Each spell (or each component of each spell) which causes damage or healing has a spell power coefficient which determines how much damage or healing the spell does.

Sources of spell power
Spell power is found primarily on trinkets as an on use effect, but can also be increased with potions.

Additional notes

 * Spell power does not apply to bandages or potions.

History
Prior to patch 3.0.2, the attribute known as spell damage would only increase the effects of damaging spells. Healing spells were only increased by the healing attribute. As of patch 3.0.2, spell power now affects both types of spells. Nevertheless, there are items which would increase your bonus healing independently of damage spells. Similarly, there are items which grant bonuses only to certain magic schools such as Fire or Nature.

As of patch 4.0.1 the ability to downrank has been removed. Spell power has also been removed from Wrath of the Lich King gear (except weapons and trinkets) and gems, but casters now gain 1 spell power per point of intellect. Spell power on gems was changed to Intellect; for example, +23 spell power became +20 Intellect.