Some attacks are so lucky or well timed that bonuses, penalties, and other factors don’t matter. The attacks simply hit.
Natural 20: When a creature makes an attack roll against a target and a 20 comes up on the d20 (this is called getting a natural 20), the power automatically hits the target. Bonuses and penalties don’t matter; the target is simply hit.
Critical Hit: An automatic hit is almost always a critical hit as well (see below).
Published in Rules Compendium, page(s) 216.