Sometimes what a voluminous book can not explain can be explained in a single sentence! It is true about yoga. Yoga is not mere a word; it is a vast subject with many dimensions.
The wise saying goes, "Yoga karmasu kaushalam!' which means, "Efficiency in work is yoga!"
What you do is not important. How you do, what you do is important!
Yoga also means union. What union? Whether union between the body and the mind? Or, the mind and the spirit? The traditionally accepted belief (or truth, depending upon the level of your spiritual progression) is the union between the Jeevatman and the Paramatman- between one's individual consciousness and the Universal Consciousness.
Yoga is a very vast subject. Attempting even an introduction to it in a small article is impossibility. Nevertheless, some important observations need to be understood when you speak of yoga.
Yoga has got something to do with the health of your physical body. Yoga has got something to do with your mental health. Yoga has something to do with your intellectual health. Yoga has got everything to do with your spiritual being. The stage ultimate! That can only be realized.
No amount of assertions, explanations, and arguments and counter arguments can take you up there. The domain of silence can only be achieved through silence! Yoga is intensely linked to breathing. It is about channelizing the energy through breath. Another intimate alley of yoga is meditation. You can not think of yoga without understanding at least some of the basic techniques of meditation! Spiritual appreciation as to the nature of the human body who controls it all the times is absolutely essential for proper understanding of various practices that make the science of yoga.
A word about Karma Yoga here: The fundamental principle of Karma Yoga is "do your duty and do not ask for the reward."
The word Yoga makes its entry at this level thinking of an individual. The correct interpretation of this principle of yoga is "Do your duty but don't ask for the rewards. You will get the right reward at the right moment!"
"Every action has a reaction and the intensity of the reaction is in proportion to the intensity of the action." Over this the scientists and the spiritualists agree. One of the rare agreements between the two contending forces, though. If you agree on this, don't you think that it is necessary for you to agree that your every action, good or bad, will be rewarded as per the proportion it deserves?
Any doubts? Be prepared to know more about yoga!