If the attack roll is higher than or equal to the defense score, the attack hits and deals damage, has a special effect, or both.
Automatic Hit: If you roll a natural 20 (the die shows a 20), your attack automatically hits.
Critical Hit: If you roll a natural 20 (the die shows a 20), your attack might be a critical hit. A critical hit deals maximum damage, and some powers and magic items have an extra effect on a critical hit.
Published in Player's Handbook, page(s) 276.