Theories of motivation are numerous and can be quite complex. Motivation tries to explain why people do what they do and what they expect out of doing those things. Issues and variables include promises of rewards, feelings of satisfaction, personal growth and time invested. Theories of motivation can range from task-specific motivations to entire lifestyles.
People under this model are motivated to complete tasks because they seek something. Some seek power, some friendship, others achievement. Whatever the goal, what is important is that tasks are means to a greater end, and that end is external to the doer.
Like the acquired needs theory, extrinsic motivation concerns goods that exist outside of the doer. Extrinsic motivation does not concern the ends of action, like acquired needs, but holds that the incentives for action come from other things, such as bosses, the promise of money or other incentives. This is a very simple model that holds that people do things because they want rewards or fear punishment.
This is a more complex approach to motivation in that the goods to be had from completing a task are internal. Usually, such internal goods are feelings of satisfaction or a general sense of competence. For such people, motivation need not be forced, but derives naturally from the internal constitution of the person. You might say that the joy of hobbies comes from intrinsic motivation.
People stick with tasks because they have already invested time and possibly money in it. In this case, it is the fear of wasted time and effort that drives the person forward. Some people remain in a task, a relationship or position because they have already worked so hard in and for it, and want to see it brought to fruition.
This is similar to the investment model, but centers around the existence of palpable progress. When you see progress being made, you have more incentive to stick with the project and see it through. Progress is a sort of internal good based around satisfaction and competence.
This is a holistic theory of motivation. It centers around the idea that doing your job well will bring happiness and satisfaction. Unlike all the other theories of motivation, this one deals not with specific tasks, but with your entire lifestyle and sense of purpose. The ultimate goal is a good, meaningful life based around success (broadly defined). This sort of success is brought about through the consistent completing of tasks, hard work and a general sense of personal competence.