Karma Yoga is literally the Yoga of Action. But here the term karma stands for a particular kind of action. Specifically, it denotes an inner attitude toward action, which is itself a form of action.
What this attitude consists in is spelled out in the Bhagavad-Gita, which is the earliest scripture to teach Karma Yoga outright.
Not by abstention from actions does a man enjoy action-transcendence, nor by renunciation alone does he approach perfection. (3.4)
For, not even for a moment can anyone ever remain without performing action. Everyone is unwittingly made to act by the qualities (guna) issuing from Nature. (3.5)
He who restrains his organs of action but sits remembering in his mind the objects of the senses is called a self-deluded hypocrite. (3.6)
So, O Arjuna, more excellent is he who, controlling the senses with his mind, embarks unattached on Karma Yoga with his organs of action. (3.7)
You must do the allotted action, for action is superior to inaction; not even your body’s processes can be accomplished by inaction. (3.8)
This world is action-bound, save when this action is [intended] as sacrifice. With that purpose, O son of Kunti, engage in action devoid of attachment. (3.9)
Therefore always perform unattached the proper deed, for the man who performs action without attachment attains the Supreme. (3.19)
– Bhagavad Gita
What Krishna is saying here is that all activity arises spontaneously as part of the program of Nature. Thus, even our thoughts are not really generated by us. Thoughts, like all processes of Nature, are simply arising.
Through Karma Yoga, whether one lives the life of a householder or of a renouncer, every action is turned into a sacrifice. What is sacrificed is the self or ego. So long as the ego presumes itself to be the author behind actions or inactions, these actions or inactions have a binding power. They reinforce the ego and thereby obstruct the even of enlightenment. Ego action or inaction generates karma.
The link between action and its feedback effects is thought to be a universal law. It appears that the karmic law is the only immutable aspect of our world of constant change (samsara). It governs the cosmos on all its countless levels, and only the transcendental Reality itself is free of this peculiar arrangement.
This teaching is closely associated with another widespread belief shared by many religions. This is the notion that the human being is a multidimensional structure or process, which does not come to an abrupt end with the death of the physical body. Diverse traditions have offered many explanations for this postmortem continuity. According to some, the surviving consciousness is clothed in a non-material body awaiting its renewed incarnation on the material plan in another physical body, or on one of the subtle planes. According to others, the ego-consciousness does not survive the death of the body, so that there is no stable transmigrating entity but only a continuity of different karmic forces.
All schools of religion agree that the mechanics of destiny on the physical plane and on any other level of existence are controlled by the quality of a person’s action or, more accurately, his or her intention. Karma Yoga is the art and science of “karmically” aware and responsible action and intention. Its immediate purpose is to prevent the accumulation of unfavourable karmic effects and to reverse the effects of existing karma.
The fulcrum of Karma Yoga is our ability to transcend all karmic necessity in our consciousness. Karma Yoga aim at the transcendence of all possible destinies in the conditional realms of the multilevel cosmos. The karma-yogin aspires to the Unconditional beyond good and evil, pain and pleasure, beyond karmic necessity and embodiment. For when the Self is realized there is only bliss, and from this position the machine of Nature cannot touch us. A Self-realized join may still suffer all kinds of adversities, but he knows himself to be infinitely above the arising qualities of conditional existence.