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.