Everyone knows the role of karma in day to day life as well as in spiritual field. There are many ways to define karma. I will try to define it in a more scientific way. There are three things 1. KARTA 2.KRIYA 3. KARMA.
KARTA = the person who performs any work.
KRIYA = the technique of work performed.
KARMA = karta + kriya (action).

It means that a person doing any act is called KARMA.
We will try to understand karma in another way. A KARTA can be any living or nonliving being eg. the sun, the earth, animals etc. are also doing their KARMA like a human. They act according to their nature. This is known as KRIYA. When there is ego or self-involvement in that act(KRIYA) of the concerned individual, then it is changed into KARMA otherwise it is only KRIYA.
If an individual feels that he is performing some act than it is known as KARMA otherwise it is the act only. In other words we will say that when person feels that he is doing some work then it is karma otherwise it is NISHKARMA ( NISHKAM KARMA).
For a yogic practitioner it is very essential to understand the theory of karma. In spiritual or yogic exercise we will always try to achieve the goal of nishkam karma. Because it is the nishkam karma only which can prevent from taking rebirths.
I have tried to explain this subject in yoga way. The practitioner should understand that there are many methods to describe any subject. The subject of karma according to SHRI MADBHAGWAD GEETA has been explained in three divisions. They are 1. Karma 2 Akarma (prohibited acts for a yogi) 3. Nishkarma (nishkam).
To achieve the goal of nishkam stage we have to pass through three main stages. 1. Early stage 2 Middle stage 3 Final stage.
Early stage Learning stage (practice of yoga in starting).
Middle stage Advanced stage ( when practitioner has visualized internal light etc).
Final stage In this stage the practitioner has completed all his exercises but has not realized his soul. It is further subdivided in 1.Matured samskar and 2. Superannuation or free from birth and rebirths.
Matured samskar means that the yogi is fit to realize his soul but he does not know the way or yogic kriya by which he will be able to merge himself in the God.
Superannuation is the final target for a yogi. By achieving this stage he has completed his duties or has merged himself in God or has become a self realized man or a perfect yogi etc.

Every act performed by a person results in sound, light and heat. These three things are responsible for internal changes in the yogi or every individual. We should carefully notice the acts of all animals eg. horse, elephant, snake, goat etc. Like human beings animals also perform yogic exercises and they also achieve their target. The God has given rights to all his creatures for self-realization or merger in Him.
The inner darkness of our consciousness is slowly and surely changed to light or realization of self by heating (burning of bad samskars of previous births) and listening to the divine sound which purifies the inner consciousness of the person making him eligible for the knowledge of the Nature or God.
I would like to conclude by saying that yogic exercise is a scientific process by which any individual can achieve his /her target by performing correct exercises (kriyas) otherwise there are chances of getting into more darkness in the form of confusion, illness, disrespect to the God or Guru etc. If it happens it will be very unfortunate because there is no remedy for it even if many births are taken. So please try to understand that you have to choose a perfect master so that he can teach you the purest form of yogic exercises.
Note: - samskars related with yoga have been discussed here only.

1. Shiv Maha Puran.
2. Shree Batuk Bhairava Upasana ch.124. Book published by Shree Venkateshwar Press Mumbai-4.
3. Bhairava Sarvasva. Book published by Peetambara Peeth Sanskrit Parishad, Datiya(M.P.).
4. Beej Yoga by Dr. Jal Singh.
5. Samskar Vigyan by Dr. Jal Singh.