When we’re talking about the Gnostics, all the rules change for what you know about Christianity. Well, not all the rules, but the big ones. The Demiurge was the name for the Gnostic deity who jealously guarded and reigned over the world, lost in a fog of its own ignorance. In the Gnostic view, this was the god of the Bible (at that time, this would be referring to the Hebrew Bible, as the New Testament was still in the process of being assembled and standardized).
Right off the bat— that definition of the Demiurge is a simplified one. The Gnostic view, one developed through decades of philosophical and theological debate, was that Wisdom, known as Sophia, was the true creator of the world, and her child was the Demiurge. The Demiurge was born into a cloud of its own ignorance, and remained unaware of Sophia’s existence above itself; it thus assumed that it was the greatest force in the universe, the creator of our world, and our natural ruler. The goal of the Gnostic movement was to reunite the spirit with the divine spark of Sophia through thought, prayer, and internal discovery.
To sum up, the Gnostics thought of the Hebrew/Christian God as a pouty infant with both hands on the wheel of a speeding universe. This high-speed baby they named the Demiurge.
One last thing— while this is what the Gnostic view of the Demiurge consisted of, there was also the idea of the Demiurge developed by Plato, centuries earlier, which referred to a more benevolent artisan of the universe. Be aware of the difference, for as you know: knowledge is power. While that thought may be laughably cliché, I use it in a form as un-ironic as possible.
Unless I’m just being really ironic.