Spell damage (or "bonus damage") is extra damage added to spells. This is the primary way that casters increase their DPS at higher levels. On items, this is typically written either as "increases damage and healing by up to X" or "+X spell damage [and healing]". There is also specific damage types available, these will specify which school of magic the bonus damage is applied to. Typically, this is simply referred to as "plus damage" or "plus [school] damage".
The actual increase that spell damage will add is determined by the spell coefficient.
The following random suffixes include bonus healing:
|..of Arcane Wrath||+ Arcane damage|
|..of Fiery Wrath||+ Fire damage|
|..of Frozen Wrath||+ Frost damage|
|..of Nature Wrath||+ Nature damage|
|..of Shadow Wrath||+ Shadow damage|
|..of Sorcery||+ Spell Damage|
|..of the Invoker||+ Spell damage and healing|
+ Spell Crit rating
|..of the Nightmare||+ Shadow damage|
|..of the Sorcerer||+ Spell damage and healing|