Man is threefold in his nature. He consists of Iccha, Jnana and Kriya. Iccha is desire or feeling. Jnana is knowing. Kriya is willing. These three fashion his Karma. He knows objects like chair, tree. He feels joy and sorrow. He wills - to do this, or not to do that.
Behind the action, there are desire and thought. A desire for an object arises in the mind. Then you think how to get it. Then you exert to possess it. Desire, thought and action always go together. They are the three threads, as it were, that are twisted into the cord of Karma.
Desire produces Karma. You work and exert to acquire the objects of your desire. Karma produces its fruits as pain or pleasure. You will have to take births after births to reap the fruits of your Karma. This is the Law of Karma.
