From early people since Hippocrates understood that babies were born a certain way, and until the turn of the century, the assumption was that they pretty much stayed that way too. People thought back then that how you were treated and what happens to you after you were born did not make a great deal of difference. This is called temperament, or your biological temperament, how you"re going to react to life in a very basic way. For most of history people believed that nature determined your take on life, your emotional take on life.
Around the turn of the century with Freud, psychologists began to learn that what happened to you after you were born was often very important. This is called nurture, learning how you"re parents treated you. What happened after this was that scientists got carried away with that idea, and sort of forgot about temperament, that the genetic basis that you were born with was also very important. One is born with a certain temperament, but your experience in your early years, in your childhood, modifies that temperament. .
One example of the nature/nurture issue would be two twins that are split up and sent into different homes after their birth. If one of those children goes into a very nice home with loving parents he will most likely grow up to reflect the behavior of the parents that raised him. This young boy could have been born with a temperament of being aggressive and feisty but the parenting that his parents gave him had a positive effect on the way he grew up. On the other hand the other twin went into a family where the neighborhood was bad and the parents were either drug or alcohol addicts. This young man might turn out to be the same as his parents or a criminal because his nurture wasn't positive and loving. .