Falling damage

In indev, infdev, and Alpha mode, you receive damage from falling. Armor can take this damage, and you are not dealt damage if you land in water that is 2 or more blocks deep, or when you are riding a minecart.

Mobs suffer falling damage as well.

Formula
Falling damage can be calculated with this formula: FD=(number of blocks x .5) - 1.5

Falling damage is only dealt after a height of 3 blocks

From a height of 23 blocks you are dealt 10 hearts worth of damage, enough to kill you if you are not a) falling into water. b) riding a minecart. c) wearing at least .5 units of armor.