The Serpent is considered to be one of Satan's earliest incarnations and the one by which he successfully tempted Adam and Eve into eating the Forbidden Fruit and thus brought upon the Fall of Man. Much like how Satan had originally been free to wander Heaven as an angel so to was the Serpent an accepted member of God's Earthly paradise (known as the Garden of Eden ) and was blessed alongside all other animals; it is believed that the Serpent originally walked on legs like other animals.
Just as Satan had caused rebellion in Heaven so too did the eternal trickster manage to cause rebellion in the Garden of Eden. As punishment for what he had done to Adam and Eve, God removed the Serpent's limbs and cursed him, alongside presumably his descendants, to slither forevermore across the ground.
The Serpent is also occassionally believed to be another form of Lilith, Adam's original wife, later succubus and Mother of Demons; this only applies to those who believe in that particular telling of the Biblical tale. According to this particular tale, after Lilith returned to the Garden of Eden to find Adam married to Eve, she vowed revenge by tempting Eve into eating the Forbidden Fruit, assuming the form of the Serpent. Although she appeared as having a snake's tail from the waist down, thus becoming an inspiration to the infamous Gorgon, Medusa.
The Serpent became a symbol of manipulation and distrust through the modern world of man.