The Darksiders 2 Skills Guide will take you through the different types of skills in the game.
The skills trees section on the skill system will give you a point for any kill that fear horseman does. You can earn points, increase boosts and basic moves. So any point that you gain for the skills will place you in three tiers. The first place of the skills ads to death to be able to repertoire and the other two will give Death the upgrade to be more powerful than ever.
The two different skill trees are harbingert tree and necromancer tree. One will give you the skills of a warrior and the other will proivde you with spellcast. Just like any upgrade in the game you will need to acquire them as you level up in the game.