The term Demiurge is a name given within some belief systems to a deity responsible for the creation of the physical universe and the physical aspect of humanity. The word is derived from the ancient Greek dmiourgos, meaning an artisan or craftsman. Although the term is used in a number of different religious and philosohpical systems (most notably Platonism and Gnosticism) the precise nature and character of the Demiurge varies considerably, from being the benign architect of matter in some systems, to the personification of evil in others.
Plato refers to the Demiurge frequently in the Timaeus as the entity who "fashioned and shaped" the material world. Plato descibes the Demiurge as unreservedly good and hence desirous that the world should be as good as possible. The reason why the world is imperfect is that the Demiurge had to work on pre-existing chaotic matter.
Gnosticism also presents this distinction between the overall "creator" and the Demiurge. However, in contrast to Plato, many systems of Gnostic thought present the Demiurge as being antagonistic to the will of the Supreme Creator, the Demiurge being focused solely on material reality and the "sensuous soul". In this context the Demiurge can be characterised as the "Satan" of Gnostic thought. In the Apocrypphon of John (in the Nag Hammadi collection), the Demiurge is characterised as "Yaltabaoth", who proclaims himself as God: "Now the archon who is weak has three names. The first name is Yaltabaoth, the second is Saklas, and the third is Samael. And he is impious in his arrogance which is in him. For he said, 'I am God and there is no other God beside me,' for he is ignorant of his strength, the place from which he had come." Some Gnostic philosophers (notably Marcion) characterise the Demiurge as being Jehovah, the God of the Old Testament, in opposition and contrast to the God of the New Testament.
The concept of the Demiurge does not reconcile easily with Christian philosophy. The Platonic interpretation seems to presuppose the pre-existence of matter (in a chaotic form) and this conflicts with the Judeo-Christian concept of an all-powerful creator who fashioned the universe out of nothingness. While the concept of Satan is well-defined in Christian literature, most theologists reject the notion that Satan (or an equivalent being) could also be the creator of the physical universe, this being the achievement of the Supreme God.
References: The Nag Hammedi Library