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Motivation has been part of the psychological vocabulary for more than a century.  In its current use, the term 'motivation' indicates a complex concept.

Here are some of the accepted definitions of motivation:

is the activation or energization of goal-oriented behavior. Motivation may be intrinsi or extrinsic. The term is generally used for humans but, theoretically, it can also be used to describe the causes for animal behavior as well.

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The processes that account for an individual’s intensity, direction, and persistence of effort toward achieving a goal

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As a rule, the term 'motivation' refers to needs, goals and desires that spur or drive an individual to action.  It is apparent from the definitions above that motivation has two components, namely movement, which implies actions, and purpose, which determines the direction of movement.

 As defined by Sternberg (1995) a motive is a want or a need that causes us to act.  Motives are caused by primary needs and secondary needs.  Primary needs are physiological and basic, for example the need for food and water; and secondary needs are psychological and derive from our primary needs, for example, the need for achievement (Murray 1938, as cited in Carver and Scheier, 2000).   

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Maslow (1968,1970) proposed that human motives are organised into a hierarchy of needs.  This hierarchical arrangement is usually portrayed as a pyramid, in which basic needs are at the bottom of the pyramid and self-actualization is the highest need.  Maslow uses this hierarchical division to show that some needs are more basic and more powerful than others and that one needs to fulfil the basic needs first before one goes about fulfilling higher-order needs. I won't bore you with explanations as I think we all know how this works by now.  Just to refresh your memory:

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The energy or impetus of a person's action can be intrinsic or extrinsic.

What is the difference?

 Intrinsic motivation means that the individual's motivational stimuli are coming from within. The individual has the desire to perform a specific task, because its results are in accordance with his belief system or fulfills a desire and therefore importance is attached to it. 

 Extrinsic motivation means that the individual's motivational stimuli are coming from outside. In other words, our desires to perform a task are controlled by an outside source. Note that even though the stimuli are coming from outside, the result of performing the task will still be rewarding for the individual performing the task. 

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Or better yet:

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Unlike intrinsically motivated individuals, an extrinsically motivated person has to be impelled.  The person himself or herself is not enthusiastic, but must be admonished or continually encouraged to perform a task.  There is little initiative or creativity within the extrinsically motivated person. 

In the primary school phase, considerable emphasis is placed on extrinsic motivation.  For example, children are encouraged to perform a task by a reward, such as a star in an exercise book.  A reward or commendation could come from the teacher and/or the parent.  Extrinsic motivation should gradually become intrinsic motivation as children grow up.

Adolescents are also exposed to extrinsic motivation.  The teacher predominantly determines adolescent's performance level by expecting work of a particular standard, which means that adolescents learn because they fear failure or because they desire to earn the approval and acclaim of peers.  Adolescents graduate to intrinsic motivation when they begin to study because they appreciate its value.  

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"Researchers have different opinions about the true value of external factors, such as a reward, commendation and approval for intrinsic motivation.  Intrinsically motivated people are sometimes swayed by external factors, but these are not decisive in their actions.  According to Sternberg (1999), society has also created many extrinsic rewards, for example, the system of education is based on grades, diplomas and various other pieces of paper that attest to what we have accomplished.  However, intrinsic motivation is irreplaceable because the motivation is primarily located in a person's will to enrich himself or herself inwardly.  From a psychopedagogical perspective, intrinsic motivation must always be the ultimate goal." 

- Quoted from: The Adolescent; 4th Edition; Pearson; Edited by E. Gouws

Since both intrinsic and extrinsic motivation can be invaluable to adolescents, teachers should strive for balance between these two types of motivation.