Yetzer Tov and Yetzer Ra are human inclinations. These are sometimes translated as a good impulse (the yetzer tov) and an evil impulse (the yetzer ra), however this should not be held as a strict definition. ---------- 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. 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. Source: JewFaq, Human Nature http://www.jewfaq.org/human.htm