"Karma theory isn't this thing where when bad things happen to you, say, you have an explanatory model for it: "It must be my karma." That's a rather unsophisticated and somewhat popular folk culture of karma. But actually, karma theory in Buddhist psychology is quite sophisticated. It's at least as complicated and as dense as Western learning theory in Western psychology. And the first principle is that if you engage in any kind of action, the engagement in that action produces karmic traces-memory traces, or karmic traces. In Tibetan, they're called bagchag, habitual tendencies or karmic traces." 

(Daniel P Brown, "Cloudless Mind 1")

See also the theory here