As quoted by the Grey Literature International Steering Committee (GLISC) (see page 1), grey literature has been defined as:
Information produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body.
In other words, the work is not tied to or controlled by commercial publishers (like the companies that publish your textbooks), who seek profit on the research or information produced. Grey literature is self-published and typically comes from:
The literature often produced from these bodies take the form of:
If you are not familiar with the terminology, the links above will take you to their definition.