A theory by Rotter (1966), stating that as a result of his (or her) past experiences, a person develops a sense that either he controls his own life and environment, and can positively or negatively affect that environment through his actions, or he cannot directly affect his life/environment.
The former is refered to as an Internal Locus of Control, the latter as External Locus of Control. The theory is longstanding and has been used as an explanation of Pavlov's Conditioning Experiments, (see also Seligman, Learned Helplessness), and distance learning drop out rates (Lynch, ASU 1997).
As a general rule, individuals with an Internal Locus of Control tend to be highly motivated, because they believe their actions will directly result in change, and individuals with an External Locus of Control tend to be less motivated and blame their circumstances on forces beyond their control. Most people lie somewhere in the middle of the continuum.
A sample of a questionnaire aimed at determining one's locus of control can be found at:
(I haven't looked in to the legitimacy of that survey, but you'll get the gist of the idea.