Karma is not the same as Predestination, Fatalism (fate), Nihilism, Luck, or God's descision.
Karma starts with intent: with intent, we create Mind, Speech, Action karma with every thought, word, and act. When the karmic cause meets with the right conditions, there shall be karmic results.
Karma is as such: every seed will bear its fruit, so we should constantly be mindful not to plant unvirtuous seeds lest we get unwholesome outcome. There is also a saying: Bodhisattvas care about the Cause, Sentient beings care only about the (karmic) results.
Generally it is quite easy to understand the concept of karma, but it would be impossible to go into detail unless one is a Buddha. Only the Buddha can completely comprehend karma completely.
This is only a general overview there are a lot of things, so I recommend people to read this webpage:
http://www.buddhanet.net/e-learning/karma.htm