Ha-satan is Identified with the Yetzer Ra:

In Judaism, ha-satan (the adversary) is mostly identified with the yetzer ra, but also identified with one who leads astray, then brings accusations against man.  Its chief functions is those of temptation, accusation and punishment.  Under the control of G-d it acts solely with divine permission to carry out its deeds.

As we see in the book of Iyov, ha-satan's function is described as that of testing the sincerity of men's characters.  Ha-satan is at all times under the control of G-d and keeps within the limits which G-d has fixed for it.

In Talmudic literature, ha-satan's function is to strengthen man's moral sense by lending him into temptation.  

"Satan" is NOT a proper name referring to a particular being who is the antagonist or rival of G-d, as Christianity erroneously teaches.   In its original application, in fact, it is a common noun meaning an adversary who opposes and obstructs. It is applied to human adversaries in 1Shmuel 29:4; 2Shmuel 19:23; 1Melachim 5:18; 11:14, 23, 25, and its related verb is used of prosecution in a law court (Tehillim 109:6) and the role of an antagonist in general (Tehillim 38:20, 21; 109:4, 20, 29).

The angel who was sent to obstruct Balaam (BaMidbar 22:32) was evidently chosen, as a "satan" (le-satan), and perhaps the consonants l-s-t-n are rather to be read as the infinitive liston, "to oppose or obstruct." (Ency. Judaica)

While the evil impulse is strong, Judaism believes that a person can choose to overcome it. This is the concept of free choice, which is basically the purpose of our existence:   To choose good over evil. The teachings of the Torah are referred to as the antidote to the yetzer hara.  Similarly, Ben Sira (21:11) states:  "The man who keeps the Torah controls his natural tendency."

"Man is the creature created for the purpose of being drawn close to G-d. He is placed between perfection and deficiency, with the power to earn perfection. Man must earn this perfection, however, through his own free will... Man's inclinations are therefore balanced between good (Yetzer HaTov) and evil (Yetzer HaRa), and he is not compelled toward either of them. He has the power of choice and is able to choose either side knowingly and willingly..." - Aish HaTorah