The word karma comes from the sanskrit root kr, which more or less means “to do”.
Every single action enacted by us will provoke a reaction of similar nature. Therefore it is a law of cause and effect.
Both of them, cause and result, are denoted with the same word, to indicate their indissolublity.
Those who are intelligent will not live in ignorance, keeping on acting blindly without calculating the nature of their doings. Virtuos actions will give pleasurable reactions, criminal actions will result in pain.
But the conclusive aim of life is not to harvest very good effects for enjoyment in this world, which is temporary, but to find those actions which will not give results whatsoever.
The only karma exempt from reaction is devotional service, or spiritual action. This will not give any karma and will carry you back to the eternal kingdomn of Godhead, our original and true home.