karma defined..

karma....is one of the most amazing words that sums up the complete human living.
it is a sanskrit word which means.... what you give is what you get.

why did our predecessors say ''be good.'' ?
that's only because of the fact that your deeds are somewhat like a pingpong ball. if you aim at the ground, it'll aim back at you as high or even higher. so, if your deeds are good, you've a good life going out there.
karma isn't deep philosophy. it's just a small understanding of our life, and how to make it better.

i can give you a strange analogy between science and philosphy..(it's upto you to understand)

according to thermodynamics, ''energy can neither be created nor be destroyed, it can only be converted from one form to another.''
karma believes in something similar...''We all do something every minute of our lives, the consequences of which we cannot escape. The consequences of your deed depends upon it's quality. Whether it is going to be good or bad, is decided by the nature of your deed.''
for example, if you do something bad, the same bad thing might just not happen to you. it might be equally bad in some other way. anyhow, you never escape.
of course, the same applies to doing good, too

so, do what you think is good, and something that's good in your opinion, invariably enters your destiny.