Samael, also spelled as Sammael, is a powerful archangel who has had many roles - the most important being that of destruction and death as well as he is an accuser and a seducer - in many ways this makes him similiar to Lucifer, but unlike the almost universally evil depictions of Satan, Samael is regarded as both good and evil. In Hebrew lore, Samael is the prince of demons, and he is known as "The Venom of God", and is the executioner of death sentences decreed by God.
Depending on tradition, Samael can be a fallen angel or he can be seen as an angel who is still loyal to divine law but enforces the darker side of faith (in either form he can be considered an alter-ego of Satan or a unique entity in his own right). He could be the embodiment of corruption and amorality.
A trait of Samael different from Satan is that even when depicted as a fallen angel he is said to be under the command of God - as opposed to being a direct enemy.
In Jewish lore, Samael was said to be the Serpent who tempted Eve into sin. He even seduced and impregnated her with Cain. Samael then became the consort of Adam's first wife, Lilith, who is also his uncircumised sexual partner. He created with her a host of demon children, including a son, Sariel, the "Sword of Samael". God feared that the Samael and Lilith would plague the world with their heinous progenies, and castrated the corrupt angel.