| GM Rednal |
Caster Level (CL) is the power of your character as a caster, and usually - but not always - equal to your level in a given casting class. For example, if you're a 6th level Wizard, your Caster Level is 6. This affects many of the effects of spells. For example, the Shocking Grasp spell lets you deal 1d6 damage per Caster Level, to a maximum of 5d6 damage. Caster level also affects the range of many spells - the higher your caster level, the further you can cast it, which can be pretty handy. Many spells use caster level for one or more effects, and how they do so is described in each spell's entry.
Spell Level refers to the numerical level of a given spell, going from 0 (weakest) to 9 (strongest). Characters who use magic get higher-level spells as they gain new levels in that class.
You can choose to cast a spell at a lower caster level than your maximum (if, for example, you want to avoid killing people), but your caster level has to be at least the minimum required to cast the spell in the first place (and that depends on your class). Our Wizard, to go back to him as an example, can't cast a 3rd Level spell like Fireball at anything less than CL 5 because that's his CL when he first gets access to them.