Your caster level (CL) is the level in your casting class, thus it may be different from your character level in the case of a multiclass character.
Many spells have a maximum caster level, i.e., when your caster level increases beyond the spell maximum, the spell effect does not increase anymore (e.g., Cure Light Wounds reaches its maximum effect at CL 5).
Note that in PnP, a Paladin or Ranger has a caster level equal to class level divided by two (but zero until level four, since they cannot cast spells before that). DDO does not follow that reasoning, so Paladins and Rangers start at caster level 4 immediately once they gain any spells. (i.e. when they reach level 4 in that class.)
The caster level of spell-like abilities granted by race, feats, or epic destinies is usually equal to your character level. These SLAs can still benefit from effects that increase CL.
Effects that increase caster level of spells and SLAs
- Item enchantments
- Illusory Death: Illusion and Necromancy spells
- Might of the Abishai item set: +1 / +3 to evocation caster level
- Spell Augmentation: increases arcane caster level
- Class / race abilities
- Song of Arcane Might: +1
- Sorcerer Savant prestige enhancements: +1 per core ability to spells of an element
- Angel of Vengeance enhancements - each core ability: +1 Caster Level and +2 Maximum Caster Level with Nimbus of Light, Searing Light, Sun Bolt, Cometfall, Divine Punishment, Flame Strike, and Fire Storm.
- Intense Faith: +3 to Fire, Force, Light, and Physical damage spells
- Draconic Knowledge: +1 to spells of selected element
- Season's Herald enhancements
- Divine Disciple enhancements: capstone, T5
- Radiant Servant enhancements: healing spells
- Runic Efficacy: +2 for electric spells
- Artificer Knowledge Potions: +2 for curative infusions
- Epic abilities
- Epic Destinies: +1 per core ability of caster destinies (+5 for a capped active destiny)
- [Color] Dragon Spell Knowledge: +3 for spells of an element
- Master of [Spell School]: +3 for spells of a spell school
Items that produce spell effect
A handful of effects exist that increase the caster level:
- Arcanotechnician capstone
- Artificer Dilettante
- Artificer Knowledge
- Clickies on quarterstaves use the caster level of the character, if it's higher than clicky's caster level.
- FvS tests (2018): ,