When you roll a natural 20(or if you hit within your weapons threat range on the first roll) or natural 1 on a d20 roll, reroll with a +20 or a -20 respectively.
Success on a d20 roll by at least 20 is a critical success, while failure by 20 or more (from the lowest DC rolled against) is a fumble.
Critical success may be critically successful more than once (EG double critical hit for x3 damage with an x2 weapon if you succeed by 40 or more)
Fumbles may stack if you fail by 40 or more
Touch attacks must beat normal AC rather than touch AC by 20 to crit, though a roll within the threat range need only beat touch AC by 20
DR and resistances stack unless otherwise specified
Effects that ignore resistances instead ignore the first 50 points of resistance, then 50% of the remainder
Effects that ignore immunity are treated as a -100% per effect to the targets immunity, minimum of 0% immunity.
Intimidate is opposed by a will save
Immunity to a condition or effect (anything but a damage type) is treated as a +20 on all saves against it, and saves are always allowed