Many times, it's important to determine not only if you succeed or fail, but also how spectacularly you succeed or fail. Exceptional results—either good or bad—can cause you to critically succeed at or critically fail a check.
Critical Success
A critical success is achieved when a check meets or exceeds the result of a check's DL or the AC of a target by 10 or more. When this check is made to deal damage, like an attack or spell effect, it is usually referred to as a critical hit.
Critical Hit Damage
When you critically hit, i.e. have a critical success on a check that deals damage, the damage is doubled after all positive modifiers have been applied.
Natural 20
If you roll a 20 on the check, the result is one degree higher than it should be. This usually results in a critical success, but can result in a success or a failure if the DL is very high.
Critical Failure
A critical failure is achieved when a check fail to meet the result of a check's DL or the AC of a target by 10 or more.
Natural 1
If you roll a 1 on the check, the result is one degree lower than it should be. This usually results in a critical failure, but can result in a only a failure or a success if the DL is very low.