Different posters have captured the meaning of Karma from different angles.
As someone said, the notion of “Karmic justice and retribution” is a Western (mis) interpretation, a hippie interpretation
As flycow said, The laws of Karma in both Hinduism and Buddhism actually refer to the inherent nature within the universe. In general, goodness will lead to goodness and evil will beget evil. In other words, it is as redundant as saying: “If you do a good thing, there will be a good result”.
But, noname hit on a philosophical concept derived from the laws of Karma, the Karma-yoga or Karma-marga, which is touted in Hinduism as a path to salvation. Karma does approximately translate to action. Essentially, the Gita speaks of how actions can create desires which creates attachment causing misery etc. Deriving from the notion that the real doer is not you but the Supreme Being, The Gita advises Man* to offer all actions to God in order to escape the laws of Karma. By not looking at the results of the actions, and by performing one’s actions and duties without attachment and in the name of God, one can attain salvation. This is the Karma-marga. Yes, Krishna asked Arjuna to perform his actions without attachment, to perform his karma, not dharma.
Note that the “from action stems suffering” idea is a core component of Buddhist thought.
Diogenes spoke of how individuals with bad karma re-incarnate as dung beetles! Here we see an application of the laws of Karma to decide the quality of birth. Good actions will help you lead a better life in the next birth. Bad actions will lead to a poorer rebirth. But, the key point is that the cycle of birth and re-birth is implicitly assumed. That is, whether Man performs good or bad karma, he may still not attain salvation, i.e., escape the cycle of birth and death. Good karma may give him a good life but he will continue to be born again and again and again… Which is why the Gita preaches that Man shouldn’t get attached to his actions and urges him to perform actions without anticipating results. By following this stringent Karma-marga, s/he can attain salvation.
Finally, there are subtle differences amongst Hinduism, Buddhism and Jainism (and may be other Eastern religions?) in how they understand “karma” … which makes the word even more complex.
*No gender bias implied