Volition.

སེམས་པ་
Tibetan: Sempa
English: volition, to will, intention, a choice

The variety of the world arises from karma, 
Karma is volition and that which is produced through volition.
Volition is mental action,
it gives rise to two actions, bodily and vocal action. 
~Vasubandu

Our lives are created through action, both the individual and collective.  The key to understanding karma is to understand that it is a choice.  We made that decision and we are responsible for its result. Our intention gave rise to our actions and our actions determine our reality.  

Karma is simple.  It is living with the results of our choices that is not.