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
- Items with Spell Augmentation enchantment: increases arcane caster level
- Might of the Abishai item set: +1 / +3 to evocation caster level
- Song of Arcane Might: +1
- Sorcerer Savant prestige enhancements: +1 per core ability to spells of an element
- 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
- Epic destinies: +1 per core ability of caster destinies
- other ED abilites, TBD
Items that produce spell effect
The caster level of items (scrolls, wands, clickies) is usually the minimum level necessary to cast the spell.
A handful of effects exist that increase the caster level: