Karma is the Hindu and Buddhist philosophy according to which the quality of people’s current and future lives is determined by their behavior in this and in previous lives. Good behavior is rewarded with goodness in life, and bad behavior is punished.
It is something like God: it takes one to be a believer to experience it and be convinced of it.
In today's world though, the applicability of karma is limited only to the current life, since future lives are more a matter of speculation.
A good intermix of right and wrong actions may help you establish a theory of karma for your personal use.