A one-sentence description of Gnosticism: a religion that differentiates the evil god of this world (who is identified with the god of the Old Testament) from a higher more abstract God revealed by Jesus Christ, a religion that regards this world as the creation of a series of evil archons/powers who wish to keep the human soul trapped in an evil physical body, a religion that preaches a hidden wisdom or knowledge only to a select group as necessary for salvation or escape from this world.
http://www.earlychristianwritings.com/gnostics.html

Some Gnostics taught that the world is ruled by evil archons, among them the deity of the Old Testament, who hold captive the spirit of humanity. The heavenly pleroma was the center of the divine life, and Jesus was interpreted as an intermediary eternal being, or aeon, sent from the pleroma to restore the lost knowledge of humanity's divine origin. Gnostics held secret formulas, which they believed would free them at death from the evil archons and restore them to their heavenly abode. See Valentinus for typical Gnostic teaching on the pleroma.
http://www.infoplease.com/ce6/society/A0821080.html