When taking combat or direct damage, this is not the same as loss of life, right? Yes, your life total goes down. But some cards have activated abilities when "loss of life" happens. For example, if I get dealt combat damage, this would not trigger a "loss of life" ability, correct?
I'm not sure to which card you're referring, but consider the wording of Archfiend of Despair:
At the beginning of each end step, each opponent loses life equal to the life that player lost this turn. (Damage causes loss of life.)
So combat damage and direct damage do cause loss of life. One of the main differences is that there are several cards preventing combat and/or direct damage, but only very few prevent loss of life (usually worded as 'your life total can't change').
Damage dealt to a player (usually) causes loss of life:
119.3a Damage dealt to a player by a source without infect causes that player to lose that much life.
So an opponent taking damage from something does trigger the ability on, say Exquisite Blood.