In the beginning was the Pleroma. The Fullness.The Ultimate Reality. The First Thought. The Ultimate Reality had a thought, and the Aeons, or angelic forces, were borne from this thought. The first and most revered of these Aeons was Sophia, or Wisdom. Sophia wanted to create something on her own, without the aid of the Pleroma, and thus (in some myths accidentally) created the Demiurge, also called Yaldabaoth or Samael. In shame, Sophia hid him within a cloud. The Demiurge, unaware of either Sophia his mother or the Pleroma, used his mother's energy to create the material world, in essence trapping her power within it. Thus the aim of Gnosticism is to recognize this spark of Sophia, of the divine within, and therefore escape the material world and reunite with the Pleroma.
In some sects, Yaldabaoth is seen as downright evil. In one Gnostic interpretation of the Flood Story(which I became interested in upon watching a documentary on those lame Ark-seekers earlier today), Yaldabaoth decided that humanity was growing too quickly, and essentially becoming too smart. Therefore, he decided to flood the whole earth to wipe out the intelligent people and save only the dim-witted or stupid people, the stupidest of all being Noah. However, Eve's daughter Norea learned of this, and attempted to enter the Ark. Noah would not allow her to, and so she burned down the Ark- three times. Finally, Yaldabaoth attempted to overpower her. She cried out for help, where she was rescued by Eleleth, an Aeon of truth and understanding, who reminded her that she was a daughter of the First Thought, an incarnation of Sophia, and that Yaldabaoth could do nothing to her.
Other Gnostic sects, mainly the Valentinians, saw the Demiurge not as some evil being, but simply ignorant. In their versions of the aforementioned myth, Yaldabaoth was trying to destroy his creation, but the First Thought told Noah, so that he could save anyone who would listen.
Ultimately, to me, the Demiurge and his Archons, or "Rulers", rulers of the material world and essentially synonymous with demons, are symbols of whatever holds you back, or keeps one from reaching his or her full potential in life. For a theist/spiritual person, it is those things that cause us to be caught up in the material, getting stuck in our short-term problems and day-to-day worries that distracts us from our spiritual selves and our awareness of the greater spiritual world that exists above and around us. If we can become more aware of the more important things in life, then we can control our inner "demiurges" or "rulers".