When an effect fires

On hit
Fires when one of your attacks lands. Auto attacks only; spells never trigger it.

On cast
Fires when you cast a spell.

On critical hit
Fires when any damage you deal critically hits. Works for both attacks and spells.

When you take damage
Fires when something damages you.

When you heal
Fires when you heal yourself or an ally.

On kill
Fires when you kill a monster.

On overkill
Fires when your damage exceeds what was left of the target's health. The excess is what the effect uses.

When your barrier breaks
Fires when a barrier protecting you is fully depleted.

On reflect
Fires when one of your reflections sends damage back to an attacker.

Every Nth hit
Counts your attacks and fires when the count is reached. Not every attack counts - see Attack Kinds.

Every N seconds
Fires on a timer, whatever you are doing.