A spell’s power often depends on its caster level, which is equal to the spellcaster's class level in the class they are using to cast the spell. If a character only has one character class (i.e. they have not multi-classed or dual-classed their character), then their caster level, class level, and character level will all be the same. If a character is dual-classed or multi-classed, a caster level is only equal to the class level of their spellcasting class. It is possible to have more than one caster level, if you have dual- or multi-classed into more than one spellcasting class. The caster levels of different spellcasting classes do NOT stack, unless you have an ability, feat, or feature that specifically states that they do.
You can cast a spell at a lower caster level than your own if you wish, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the adjusted caster level.
In the event that a class feature or other special ability provides an adjustment to your caster level, that adjustment applies not only to effects based on caster level (such as range, duration, and damage dealt), but also to your Caster Check to overcome your target’s Spell Resistance and to the caster level used in dispel checks (both the dispel check and the DC of the check).