On the question of human nature, as in most areas of abstract belief in Judaism, there is a lot of room for personal opinion. There is no dogma on the subject, no required belief about the nature of humanity. There are a variety of contrary opinions expressed on the subject, and one is no less a Jew (and no less a good Jew) for disagreeing with any or all of these opinions. Nevertheless, there are certain ideas that seem to reflect the majority opinion in Jewish thought that are worth discussing.
In the Image of G-d
The Bible states that humanity was created in the image of G-d, but what does it mean to be created in the image of G-d?
Clearly, we are not created in the physical image of G-d, because Judaism steadfastly maintains that G-d is incorporeal and has no physical appearance. Rambam points out that the Hebrew words translated as "image" and "likeness" in Gen. 1:27 do not refer to the physical form of a thing. The word for "image" in Gen. 1:27 is "tzelem," which refers to the nature or essence of a thing, as in Psalm 73:20, "you will despise their image (tzel'mam)." You despise a person's nature and not a person's physical appearance. The word for physical form, Rambam explains, is "to'ar," as in Gen. 39:6, "and Joseph was beautiful of form (to'ar) and fair to look upon." Similarly, the word used for "likeness" is "damut," which is used to indicate a simile, not identity of form. For example, "He is like (damuno) a lion" in Ps. 17:12 refers not to similar appearance, but to similar nature.
What is it in our nature that is G-d-like? Rashi explains that we are like G-d in that we have the ability to understand and discern. Rambam elaborates that by using our intellect, we are able to perceive things without the use of our physical senses, an ability that makes us like G-d, who perceives without having physical senses.
The Dual Nature
In Genesis 2:7, the Bible states that G-d formed (vayyitzer) man. The spelling of this word is unusual: it uses two consecutive Yods instead of the one you would expect. The rabbis inferred that these Yods stand for the word "yetzer," which means impulse, and the existence of two Yods here indicates that humanity was formed with two impulses: a good impulse (the yetzer tov) and an evil impulse (the yetzer ra).
The yetzer tov is the moral conscience, the inner voice that reminds you of G-d's law when you consider doing something that is forbidden. According to some views, it does not enter a person until his 13th birthday, when he becomes responsible for following the commandments. See Bar Mitzvah.
The yetzer ra is more difficult to define, because there are many different ideas about it. It is not a desire to do evil in the way we normally think of it in Western society: a desire to cause senseless harm. Rather, it is usually conceived as the selfish nature, the desire to satisfy personal needs (food, shelter, sex, etc.) without regard for the moral consequences of fulfilling those desires.
The yetzer ra is not a bad thing. It was created by G-d, and all things created by G-d are good. The Talmud notes that without the yetzer ra (the desire to satisfy personal needs), man would not build a house, marry a wife, beget children or conduct business affairs. But the yetzer ra can lead to wrongdoing when it is not controlled by the yetzer tov. There is nothing inherently wrong with hunger, but it can lead you to steal food. There is nothing inherently wrong with sexual desire, but it can lead you to commit rape, adultery, incest or other sexual perversion.
The yetzer ra is generally seen as something internal to a person, not as an external force acting on a person. The idea that "the devil made me do it" is not in line with the majority of thought in Judaism. Although it has been said that Satan and the yetzer ra are one and the same, this is more often understood as meaning that Satan is merely a personification of our own selfish desires, rather than that our selfish desires are caused by some external force.
People have the ability to choose which impulse to follow: the yetzer tov or the yetzer ra. That is the heart of the Jewish understanding of free will. The Talmud notes that all people are descended from Adam, so no one can blame his own wickedness on his ancestry. On the contrary, we all have the ability to make our own choices, and we will all be held responsible for the choices we make.