Falling Damage: The basic rule is simple: 1d6 points of damage per 10 feet fallen, to a maximum of 20d6.
Jumping to avoid damage: If a character deliberately jumps instead of merely slipping or falling, the character receives no damage for the first 10 feet and on a DC 15 DEX (Acrobatics) check he receives no damage for the first 20 feet and lands on his feet. Thus, a character who slips from a ledge 30 feet up takes 3d6 points of damage. If the same character deliberately jumped, he takes 2d6 points of damage. And if the character leaps down with a successful Dexterity (Acrobatics) check, he takes only 1d6 points of damage from the plunge.
Falling onto Soft Surface: Falls onto yielding surfaces (soft ground, mud) also ignores the first 1d6 points of damage. This reduction is cumulative with reduced damage due to deliberate jumps and the Athletics skill check.
Falling into Water: Falls into water are handled somewhat differently. If the water is at least 10 feet deep, you must succeed on a DC 10 Dexterity (Acrobatics) check to enter the water without damage. Otherwise you receive 1d6 points of damage from any fall up to 20 feet of falling. Regardless of the save, you receive an additional 1d6 of damage for every 10 feet fall beyond 20 feet.
Diving into Water: Characters who deliberately dive into water take no damage on a successful DC 15 Dexterity (Acrobatics) check, so long as the water is at least 10 feet deep for every 30 feet fallen. However, the DC of the check increases by 5 for every 50 feet of the dive.
Landing on Your Feet: The character lands on his feet if he makes his Athletics check.