What is the meaning of the "Yetzer Harah" - יצר הרע? How does it compare to the סטן?
I saw the following ideas in The Encyclopedia of Talmudic Disputes and Perspectives: R. Yochanan and Resh Lakish by R. Nachman Cohen, which attempts to apply a few "big ideas" which divide R"Y and R"L as the reasons behind their many disagreements:
Resh Lakish holds that the Yetzer Hara is a creation unto itself and enters the human causing him to sin. The source is:
Resh Lakish said: A person does not transgress a sin unless he is invaded by a spirit of insanity (R. Cohens traslation- Demon of Frivolity)... (Sotah 3a)
The idea of an outside force causing sin is in line with another statement made by Resh Lakish:
The Satan, the Yetzer Hara and the Angel of Death are the same... (Bava Basra 16a)
R. Cohen posits that not everyone agrees with the idea of an outside force causing sin, but rather that the "Yetzer Hara" and Yetzer Tov" are manifestations of the human condition. While he brings many chachamim who agree to this (R' Avraham ben haRambam, Rabeinu Bachye and R' Yisrael Salanter), his sole Talmudic source is a Midrash Tanchuma (Breishis 7):
...If you say 'why did He create the Yetzer Hara...is one (now) able to do good?' Hashem said 'You make it bad! Why does a child of 5, 6, 7, 8, and 9 not sin; rather from 10 and on he empowers his Yetzer Hara'...
The Midrash continues to blame a persons inability to control his Yetzer hara on his apathy.
R. Cohen makes an assumption that R. Yochanan holds of the latter approach. He applies these different approaches to other differences between R"L and R'Y. For example, R"L held that a wicked person who repents is greater than he was before sinning, because his pure soul reached a higher level through love of G-d (Tanchuma- R"L on Yechezkel 18:27), whereas R"Y held that a soul that sins is no longer pure, but internally damaged and cannot return to its former state even through repentance (based on Breishis Rabba 21:8).
Summary: Some hold that the Yetzer haRa is an external force. Others hold it is a manifestation of the human condition.
אמר ר"ל הוא שטן הוא יצר הרע הוא מלאך המות
The Gemara Bava Basra 16. says that the Satan, the Yetzer Hara, and the Malach Hamaves are all one.
Rav Hirsch explains that it is the temptation toward the physical. As a result of this definition it is not categorically bad (nor is the yetzer tov necessarily good). Only the majority of the time does it lead people to negative/destructive things. However, when it overtook the yetzer tov, that was enough to seal the world's fate for annihilation by flood.