Gnosticism is a scholarly term for a set of religious beliefs and spiritual practices common to early Christianity, Hellenistic Judaism, Greco-Roman mystery religions, Zoroastrianism (especially Zurvanism), and Neoplatonism.
A common characteristic of some of these groups was the teaching that the realisation of Gnosis (esoteric or intuitive knowledge), is the way to salvation of the soul from the material world. They saw the material world as created through an intermediary being (demiurge) rather than directly by God. In most of the systems, this demiurge was seen as imperfect, in others even as evil. Different gnostic schools sometimes identified the demiurge as Adam Kadmon, Ahriman, El (deity), Saklas, Samael, Satan, Yaldabaoth, or Yahweh.
Jesus is identified by some Gnostic sects as an embodiment of the supreme being who became incarnate to bring gnosis to the earth. Others adamantly deny that the supreme being came in the flesh, claiming Jesus to be merely a human who attained divinity through gnosis and taught his disciples to do the same. Among the Mandaeans, Jesus was considered a mšiha kdaba or "false messiah" who perverted the teachings entrusted to him by John the Baptist. Still other traditions identify Mani and Seth, third son of Adam and Eve, as salvific figures.
Source: Wikipedia, "Gnosticism", available under the CC-BY-SA License.