Critical hits: You score a critical hit when you attack and roll a 20. A critical hit always hits the target, regardless of your to hit number (assuming the target isn't immune to your weapon). Sometimes you also score a critical hit when you roll a 19. This happens when you are proficient in the two-handed weapon style or the single weapon style. A critical hit deals more damage than a normal hit. Helmets protect from critical hits. Some creatures are immune to critical hits.
Critical misses: You get a critical miss when you attack and roll a 1. A critical miss always misses the target, regardless of your to hit number, and your next attack is delayed.
How much is your next attack delayed with a critical miss?
How much extra damage do you get from a critical hit? With magical weapons, do you also get a bonus to the extra magical damage?
What else makes you score a critical hit with a 19?
Is it possible to score critical hits with lower numbers (18, 17...)?
Are there critical rolls in any other rolls, like saving throws or thieving ability rolls? If so, how do those work?
Is there anything apart from helmets that protects from critical hits? And from critical misses?