Disclaimer: The following is but a brief clarification on what the human brain does when one's behaviour is reinforced or punished. Thorough, exhaustive, and scholarly it is not.
Summary: Punishment, reinforcement, etc. of a behaviour creates an association in the mind of the affected party between the behaviour and the corresponding punishment, reinforcement, etc., the nature of which can only be known by the affected party. Take care when reinforcing or punishing others, as you may be effecting an unwanted association.
I've noticed the behaviourist concept of reinforcement thrown around a great deal on this site, and am worried a fair number of those who frequent it develop a misconception or are simply ignorant of how reinforcement affects humans' brains, and why it is practically effective.
In the interest of time, I'm not going to go into much detail on classical black-box behaviourism and behavioural neuroscience; Luke already covered the how one can take advantage of positive reinforcement. Negative reinforcement and punishment are also important, but won't be covered here.
The Couple has a three year old son. They are worried their son ingests too few and poorly varied micronutrients. They want their son to become a world conqueror someday, so a poor diet just won't do. Their son loves DragonBox. Every time they all sit down for a meal, their son will barely consume anything, so eager is he to play more DragonBox.
He also very much likes Dragons.
The Couple decides in favor of serving their son a balanced meal consisting solely of micronutrient dense, bioabsorptive foodstuffs. They tell him, "Spawn, you must fletcherise and swallow all that is on your plate if you wish to play more DragonBox." The son understands, and acts accordingly; he eats all that is put before him so he may DragonBox.
What happens in the mind of The Couple's son when he is told thus? His brain creates a new association, or connexion, of concepts; in this case, 'eating all that is put before me' becomes associated with 'more DragonBox!' Perhaps, though, he associates 'more DragonBox!' with a different concept: 'eating green things', say, or 'eating brown things'.
In other words, one can never be certain of the precise association another creates when they are reinforced or punished.
I think I can make this explanation clearer.
Once a man named Watson wanted to investigate whether he could make a child fear something the child would otherwise not fear. He took a fluffy white rat, put it in front of a baby, Little Albert, and created a loud resounding metallic gong of a noise. After a while, Little Albert came to associate 'loud scary noise' with 'fluffy white thing' - not 'fluffy white rat'. Afterwards, when presented with a fluffy white bunny, dog, and even cotton balls, he displayed a fear response.
Humans are constantly creating associations between anything that can be conceptualised - the color indigo, Herrenhausen, toothpick collecting - anything. When one is forced to link one concept to another, through any means, an association is created; one can never know with certainty the nature of another's association.
Be careful when using reinforcement and punishment on others. Ever be diligent.
 This is called a generalised response in psychology; look into fear learning of the amygdala if keen.
 Richard Kennaway provided a quote epitomising this concern, and further elaborated upon it admirably.