jnyaanam jneyam parijnyaataa trividhaa karmachodanaa |
karanam karma karteti trividhihi karmasangraha || 18 ||

Knowledge, the known and the knower, these three initiate action. The instruments, the target of action and the doer, these three are the basis of action.
jnyaanam : knowledge
jneyam : known
parijnyaataa : knower
trividhaa : three
karmachodanaa : initiate action
karanam : instruments
karma : target of action
kartaa : doer
iti : these
trividhihi : three
karmasangraha : basis of action
The culmination of the Gita teaching is the realization that our true nature is the eternal essence, that does not act in this world, nor experiences anything in this world, since action and experience are in the realm of Prakriti. Most of us, however, are still becoming qualified for this teaching through the practice of karma yoga. Shri Krishna recognizes the need to give an in depth analysis of action for people like us. To that end, he uses this shloka to begin that topic. Each word, each term needs to be understood clearly, since the regular meanings may confuse us.
So then, what is the genesis, the birth of an action? How does an action commence? Our sense organs send a report to the mind of having seen, felt, touched, tasted or smelled something. This is the process of perception. Or, a thought about a prior perception arises in the mind. Both these processes are the same for all people in this world. Both you and I perceive a red apple in the same manner. In this case, the red apple is termed as jneyam, an object which is known. This is step one.
What happens next? Both of us see the same red apple, but you may love it, someone else may hate it, and I would be indifferent to it. This difference in our view towards the apple, our worldview in general, is due to the difference in our sanskaaras. We attach a certain meaning to objects, people and situations based on our sanskaaras. This individual vision of the world is termed knowledge or jnyaanam. It looks at the object in question and generates a sense of attraction, repulsion or indifference. This is step two.
Next, this notion of attraction, repulsion or indifference creates a modification in the intellect called the “doer”, the kartaa. It is a phantom, illusory entity which says “I want the red apple, go get it”, or “I hate this red apple, throw it away”. The doer issues these instructions to the karana, the organs of action, which then do as they are told. The action of grasping the apple or throwing it away is carried out. Note the the doer only comes into existence when there is attraction or repulsion. The jeeva is a witness, indifferent to likes or dislikes. So the doer, the organs of action and the object make up karma sangraha, the basis of action. This is step three.
When finally, the object, the target of action is consumed by the senses, another modification of the mind called the “enjoyer”, the bhoktaa, arises. It creates the notion that “I have experienced this object, and it gave me joy/sorrow”. This is the parijnyaata, the knower, mentioned in the shloka. Furthermore, a record of this experience, whether pleasurable or painful, is stored in the unconscious aspect of our personality, the causal body. This record, this samskaara, becomes the seed of future action by creating thoughts of desire in the mind, prompting further actions and experiences. This process of enjoyment of an object is the fourth step.
So these four steps taken together describe the lifecycle of an action, from start to finish.