Creatures that fall onto a solid surface take 1d6 points of damage per 10 feet fallen, to a maximum determined by the GM. In normal air on the Prime Material, the normal maximum for falling damage is 20d6. Falling in the Aether, where there is no normal air, may have no maximum limit at all, while falling in the Plane of Earth might be flat-out impossible. This may also be modified by the surface being impacted. Falling onto jagged broken obsidian shards might add +2 per die, a jumble of broken tree-trunks or solid stone may add +1 per die. Normal unremarkable 'ground' is base damage. Soft loam or deep grass might remove -1 per die. Falling allows no saving throw to reduce the damage, although you may attempt an Acrobatics check to take some of the sting off (see below). Falling damage is a form of uncommon physical damage, related to, but different from, bludgeoning damage. Creatures that sustain damage from a fall (after any mitigation they might have, such as DR) also gains the prone condition.
If a character deliberately jumps instead of merely slipping or falling, the first 10 feet of the fall inflict no damage. A DC 15 Acrobatics check allows the character to avoid any damage from the second 10 feet as well. Thus, a character who slips from a ledge 30 feet up takes 3d6 falling (physical, uncommon) damage. If the same character deliberately jumps, they take 2d6 points of falling damage. If that character leaps down with a successful Acrobatics check, they take only 1d6 points of falling damage from the plunge. In all three scenarios, since the character takes damage, they would also fall prone, unless they could somehow reduce that damage to zero through some mitigation such as DR.
A character cannot cast a spell while falling, unless the fall is greater than 500 feet or the spell is an immediate action, such as Feather Fall (Sorcerer/Wizard Spell). Casting a spell while falling requires a Caster Check to concentrate with a DC equal to 10 + (spell level x 4). Casting Teleport (Sorcerer/Wizard Spell) or a similar spell while falling does not end your momentum, it just changes your location, meaning that you still take Falling Damage, even if you arrive atop a solid surface.