Experience points (exp or xp) are awarded each time you damage a creature in category:combat. The amount of experience a given monster has is a function based on how well it does damage, how well it is armored, how long it is likely to last in combat, as well as whether it has any special abilities. In general, two mobs of equivalent difficulty will award the same number of experience points. If you do 1% damage to a monster in battle, you will receive 1% of its exp. If you are grouped, that 1% of the exp is split over the group according to a number of formulas.
It is important to note that exp is awarded for directly damaging a creature's main hit points. Since damage to limbs only does a small amount of damage to a creature's main hit points, limb damage yields small amounts of experience points. Also, indirect or time-delay damage (bleeding, starvation, suffocation, etc) will yield no experience points although some attacks from ranged weapons will.