In talking about the debate on whether nature or nurture is more important in determining the differences in behavior and personality among individuals, those who would argue on the side of nature believe that hereditary and other inherent factors are the most important factors to consider. In other words, a child is born with inherent characteristics which will determine how he or she will behave in the future. Heredity plays a big role for those who believe that the influence of nature is primary; the traits handed down in the genetic makeup of an individual are of critical importance in determining behavior and tendencies during growth and in later life.
Those who would argue that nurture is the primary influence believe that outside forces are the most important factors in determining personality and behavior. Taken in its most extreme perspective, a child is born as "a blank slate;" environment and experiences then combine to act on that blank slate, shaping personality and behavior as the individual develops.
In a nutshell, the "nature" theory says that personality and behavior are determined by influences already in an individual, whereas the "nuture" theory says that personality and behavior are shaped by the outside forces to which an individual is subjected during development.
In a simplistic but practical example of the nature versus nurture debate, imagine a man who has become a criminal. A person who believes nature is the cause will say that the man was born with certain inherent tendencies which inclined him to a life of lawlessness, whereas one who believes nurture is the cause will argue that the man became a criminal because he had, perhaps, a bad childhood.