In fact, the evolution of the Satan concept is a fascinating part of the evoilution of Hebrew/Jewish religious thought.
Let’s start by correcting a slight misstatement by Alessan. The word “satan” in the oldest strands of Hebrew did not mean precisely “enemy” but rather “accuser.” In the Really, Really Supreme Court, YHWH, C.J. presiding, the Satan was the prosecuting attorney, whose job it was to bring the iniquities of the person at trial to the court’s attention. This is the case in the legendary frame story on which the Book of Job was structured (the vast majority of the book as we have it is a sort of “Why Bad Things Happen to Good People: A Wisdom Dialogue, based on the Job story”, and not relevant to our purpose here). Satan seems to have early on moved from the role of Hamilton Burger to that of Ken Starr, a special prosecutor charged with ferreting out misdeeds. From this it was a short jump to tempter, trying to entrap the person into committing misdeeds through human weakness.
During Exile and especially Persian-Empire times, this conception shifted greatly. Mazdism (“Zoroastrianism”_) conceives of a good deity (Ahura Mazda/Ormazd), confronted by an evil lesser (but still powerful) deity (Angra Mainyu/Ahriman), who was or became viewed as a rebel against Ahura Mazda. The Satan, from being an anti-humanist functionary of the divine court, took on under this influence the role of rebel against the divine will, in short, the character of Satan as he is seen in the New Testament and in Christianity. His supposed fall from Heaven, his penchant for lies, etc., all derive from this elaborated later concept.
As noted above, it’s important not to read into the Biblical Satan the identities of other figures (Lucifer, the Morning Star, equated with a now-fallen powerful potentate), Beelzebul, etc.) In particular, it is only with the New Testament that we find the serpent of Eden identified with Satan; there is no Old Testament equivalence between the two.
Psychologically the idea of a tempter is very useful – it enables one to blame Someone Else for his or her own moral weaknesses. The Flip Wilson quote can be taken very seriously here – if “the Debbil made me do it,” then I’m not really as much to blame, now, am I? It does however frustrate the crap out of me that supposed “conservative Bible-believing Christians” make such a big deal out of Satan – they fail to recognize that they are turning the Judaeo-Christian monotheism into a pseudo-Zoroastrian dualism in doing so.