
avr |

Ability damage heals naturally - one point per day of normal activity, two points per day of full rest, double either if someone with the heal skill provides care - or can be healed by spells like restoration.
Ability drain (blood drain normally does ability damage not ability drain, BTW) requires spells to heal. It does not heal naturally.

vhok |
ability damage also only affects someone on even numbers.
example
14 str takes 1 ability damage they have NO PENALTY YET and still have a +2 str bonus. it does not make their str 13, ability damage has NO EFFECT until they take 2/4/6/8.
14 str takes 3 ability damage they will still have a +1 str bonus it has to hit 4 ability damage before they hit +0
2 ability damage is the same as 3 ability damage 0/1 2/3 4/5 are all the same negative effect. the only difference is how many days it takes to remove it all.
ability drain affects them right away
example
14 str takes 1 ability drain they are now at 13 and have a +1 bonus
blood drain is usually ability DAMAGE not drain