From the Internet Encyclopedia of Philosophy http://www.iep.utm.edu/gnostic/
"Gnosticism (after gnôsis, the Greek word for knowledge or insight) is the name given to a loosely organized religious and philosophical movement that flourished in the first and second centuries CE. The exact origin(s) of this school of thought cannot be traced, although it is possible to locate influences or sources as far back as the second and first centuries BCE, such as the early treatises of the Corpus Hermeticum, the Jewish Apocalyptic writings, and especially Platonic philosophy and the Hebrew Scriptures themselves."
What you tend to find in gnosticism is a belief that the mind is good and the body is bad. They search for enlightenement which is only available to a few. The Nag Hammandi library has lots of the texts that had been lost or destroyed so is really important to understand gnosticism which was only known by what its opponents had written about it.
There is some good stuff on the gnostic gospels on old rejesus as they did a lot of work on it for the Da Vinci Code film. http://old.rejesus.co.uk/davinci/gospels_intro.html
Christianity gnosticism is outside mainstream Christianity. I find gnosticism facinating and complex and it is a while since I studied it but as far as I can make out one of the foundational differences between gnosticism and Christianity is that the world in gnostistic thought is corrupt and the point of enlightenment is to find a way for the soul to escape it. Mainstream Christianity holds that the world and creation is good; it is fallen but it is good.
Heard an interesting Radio 4 programme on the Gnostics a few weeks back. Does anyone know what they are all about and how the Christian Gnostics differ from conventional Christians and where the Nag Hammadi differ from the New Testament etc?