Once there was a man who was born blind. He was the eldest son of a king, but his blindness denied him the throne. He was blessed with a hundred sons- but they were all butchered in war, while he lived.
This man was Dhritrashtra- the father of the Kauravas.
"How cruel! How unfortunate! What a sad fate!" you may think.
But consider this: In a previous life, Dhritrashtra was a hunter. Once he saw a bird nest with 100 newly hatched chicks. Out of sport, he killed all the chicks, blinded the parent birds and left. Now- didn't he get what he deserved?
Karma-the law of nature
There is a simple law that exists in the material world:
"Every action has an equal reaction"
What goes around, comes around.
You do good and enjoy the positive fruits, you do bad and you get punished. Ofcourse good and bad are also clearly defined in the scriptures-your own notions of good and bad are not going to change anything. Ignorance of the law too is no excuse.
Free will Vs destiny
Destiny is chartered out by your activities in a previous life. For eg previous karma determines your looks, your family background, your health and your intelligence. It is not random chance that determines why someone was born in the Gandhi family and why someone else was born to a beggar on the street. Its karma....
And free will? It is the choices you make while using the facilities given to you.
For eg: If you are born in a powerful family- you have a choice. How are you going to utilize the power given to you? If you chose to misuse your position in exploiting others, this facility will definitely be taken away in the next life.
We can chose how to act- the results of our actions are determined by karma. And our actions are determining the path our next life will take.
Heaven and hell
The Vedic understanding of heaven and hell is more complete than the Biblical understanding.
When you accumulate a lot of good karma, you get to go to the heavenly planets to reap the benefits. There are numerous heavenly planets and they have numerous facilities for enjoyment. For eg, you will get a beautiful body with a very long lifespan. They say that the most beautiful woman on earth looks like a frog compared to the heavenly damsels. There is great wealth and so much power. You could be made the head of wind, or of water or even become the King of heavens- Indra. But when your accumulated piety finishes off- you must once again return to earth.
Similarly too many sinful activities take you to hell. There you are punished for the stipulated time period and then sent back to earth. Recently I was reading the descriptions of hell given in the Srimad Bhagvatam. Quoting directly:
"One who appropriates another's legitimate wife, children or money is thrown into the dark hell known as Tamishra. There he is starved, given no water and is beaten until he falls unconscious"
23 hells are thus described and there are many more which have not been elaborated. Thus no crimes go unpunished.
As you can see karma keeps us bound. We keep getting different bodies, in different situations, as per our desires and our karma. Karma is compared to a dense network of banyan trees- roots from one tree lead to the growing of another tree until you can find no beginning and no end.
This network can be cut only by taking the shelter of Lord Hari. We cannot be inactive- action must be done. Hence action is done for God. When you work for God by offering the results of your activities to Him- then it is 'akarma'- karma that brings no reaction.(Eg, before eating, the cooked food is offered to the deity, time and money are utilised in His service etc)
Being liberated from these karmic chains we can then go to our true home- the spiritual world which is beyond both heaven and hell.