Lifelink
Keyword abilities
Damage dealt by this source also gains its controller that much life.
How it works
Lifelink is a static ability that modifies damage: when a source with lifelink deals damage, that source's controller gains that much life at the same time the damage is dealt.
It applies to all damage from the source — combat, abilities, or burn — not just attacks.
Common confusions
“Is the life gain a separate trigger you can respond to?”
No. The life gain happens simultaneously with the damage, as part of the same event. There's nothing to respond to in between.
“Two instances of lifelink.”
Redundant. Lifelink isn't cumulative — having it twice gains life once per damage event.
“Lifelink damage that's also lethal/deathtouch.”
Still gains the life. Destroying the creature and gaining the life happen from the same damage event.