Karma means action or deed. It is derived from the Sanskrit word Kri which means 'to do.' Any
physical or mental action is karma. Technically, the word also means the
effects of actions. In connection with meta-physics, it sometimes means
the effects, of which, our past actions were the causes.
Karma yoga purifies the heart by teaching you to act selflessly, without
any thought of gain or reward. By isolating yourself from the fruits of
your actions and offering them up to God, you learn to sublimate the ego.
To achieve this, it is helpful to keep your mind focused by repeating a
mantra while engaged in any activity.
Karma yoga joins both physical and mental aspects of the Hindu philosophy.
The essence of karma relies heavily on action; it theorizes past actions
that reckon consequences and affects one's position and progress in life.
By doing selfless service you will purify the heart and develop humility,
sympathy, tolerance, mercy, etc.
You can practice all the other yogas with ease, but the most difficult
one is Karma Yoga, because the ego will pop up at every moment. But the
secret is to forget yourself, to place yourself last and put others first.
Then Karma Yoga will be easy. The aim of Karma Yoga is to gain freedom
from the bondage of karma, which restricts and hinders dynamic, creative
and constructive expression in life, and perform your duty.