From what I know and use so far,
Executioner applies to what nets you a kill on that turn from melee to ranged to spell.
Opportunist talks about melee attack. When you try to run away from enemy with opportunist, you can see a circle around then and if you leave that area, they get a free hit.
Savage uses whatever you currently has as a crit chance. Rage got nerfed to silence the caster so you can't abuse that with guaranteed spell crit anymore.
You can test most of them in the game master mode as well by creating 2 different char with whatever talent and ability you want to give and try them out in combat or pre-combat. It's best you visit that when you plan on what to play.
