Gain on fatality and gain on kill

Can someone explain the difference? someone posted that this topic has been covered various times, but cant manage to find the post. Can someone link it please, otherwise can you just be kind and explain the difference. Thank you

fatality is death through rune or charge attacks

Oh wow I never knew that. We really need a codex or something to properly explain these systems. There are still probably many systems that I don’t understand because there is not enough explanation.