Karma isn't personal. Not in the way westerners often believe: that it is tied explicitly with the person generating it. The righteous do not necessarily benefit from any good karma generated, except to rub some of the rust off the copper.
Karma explains how actions effect things. That is basically all it explains, without getting deeper into other details.
All actions have a result, yes? That is karma.
You can do nothing to generate karma in this lifetime and still have karma. Karma is in essence why you are here and not free. (aka moksha, liberated...) It's perhaps improper to think in terms of good and bad karma, but rather that _all_ karma is a manifestation of attachment. The only good dharma does in the case of not piling up more debt in the system, so to speak. Enlightenment means you paid the debt in full and owe nothing, because there is nothing for you to lust after and you are incapable of harming others.
Debt is perhaps the best analogy to use when explaining karma to others so they cannot inject other concepts into it. It's not 'doing the right thing' so much as it is 'doing the only thing' that gets you out of hock. Until the debt is paid, you owe time served in reincarnations.