[Kristoffer O]

As Arralen says + means higher dmg with higher lvl. THe amount varies from spell to spell, but is about +1 for spells with dmg 1-10, +2 for spells with dmg 10-20. Some spells have higher bonuses, but that is rare (thunder strike IIRC and banefire). In the case of fatigue dmg it might be +5 / lvl (ghost grip).