Christian Beliefs about Human Nature
One of the most prominent messages of the Bible concerns the nature of human beings. Fundamental to the Christian understanding of human nature is the belief that the first humans were created in the image of God (also called imago Dei). This is the teaching of Genesis 1:26-27, which states, "Then God said, 'Let us make man in our image, in our likeness and let them rule over the fish of the sea and the birds of the air, over the livestock, over all the earth, and over all the creatures that move along the ground. So God created man in his own image, in the iamge of God he created him; male and female he created them.'" (Learn more about creationism here.)
The nature people were created with was good (cf. Gen. 1:31), but according to the Bible people were given a free will with which to chose for God or against him, and in the Garden of Eden Adam and Eve were persuaded by the serpent to rebell and as a result sin entered the world (cf. Gen. 3:1-19) and afflicted the human race (cf. Rom. 3:23). (Learn more about Satan here.)
Below you will find more discussion on the fallen nature of people as well as original sin. (Go here for a more in-depth article on the image of God, imago dei.)
A Fallen Nature
Most scholars agree that the Bible teaches that the of Adam and Eve's sin in the Garden of Eden resulted in the entrance of physical death into the world.  Beyond that, Christians views differ as to the effect Adam's sin had on the rest of humanity. Most have taught that when Adam fell and was cast from the Garden of Eden, permanent damage was done to the human soul, such that every human being since Adam is born with a tendency towards sin. (Learn more about the Bible here and God here.)
In addition, sin is universal, according to the Bible. Every human being has fulfilled the inborn tendency and actually committed sins. This concept is most fully articulated in the New Testament by the Apostle Paul, who declared that "all have sinned and fallen short of the glory of God. there is no one who is righteous, not even one" (Rom. 3:23) (Learn more about the Apostle Paul here.)
Many Christians have also taught the doctrine of "original sin," in which all humans are born not only with a predisposition to sinful behavior, but with an inherently sinful nature. The result is that every person is born deserving eternal damnation, whether or not they have actually committed a sin yet. The doctrine of original sin has been especially emphasized by St. Augustine and most of the Protestant Reformers.