So one of the things this game needs BADLY is tool tips for everything. Like I dont know what the difference between Siphon and Drain are and the game doesnt do any job of describing it to me.
Drain makes the affected stat go down (possibly over time if you set it up like that) basically used for denying something its mana/rage.
Syphon is the same as drain but the drained amount is added to the person who used the skill (aka stolen) and can be set up for health also.
But i do agree that some UI work is needed to explain.