It's a belief that there is a body of secret knowledge that one can come to know through careful study of the words of a sage. Christian Gnostics studied the words of Jesus and searched for the meaning hidden behind the words.
The doctrines of certain Prue-Christian pagan, Jewish, and early Christian sects that valued the revealed knowledge of God and of the origin and end of the human race as a means to attain redemption for the spiritual element in humans and that distinguished the Demiurge from the unknowable Divine Being.
Gnosis means knowledge and is a reference to the Light of Consciousness or inner light.
The Gnostics are the ones who were terrorized by the Romans and probably killed off by them... and thus, this may be the reason we hear the Pauline gospel of blood for sin these days, rather than the Original Gospel of Light that Jesus actually taught. Click on my avatar for an explanation of my comment.
Gnosticism is the more near form of religion to the teachings of Jesus himself.
The first Christians where all Gnostic's but the catholic church was not happy with that so it removed most of the material about this and replaced it with a personal cult (the cult of Jesus) when Jesus never wanted to be worshiped and always only asked worship for the Father and only for the Father.