The license is designed for software documentation and other reference and instructional materials. It stipulates that any copy of the material, even if modified, carry the same license. Those copies may be sold but, if produced in quantity, have to be made available in a format which facilitates further editing. Wikipedia is the largest documentation project to use this license.
Many people and groups, notably the Debian project (based on their Debian Free Software Guidelines), consider the GFDL a non-free license, due to both the usage of "invariant" text that cannot be modified or removed, and the well-meaning but overly-broad prohibition against DRM systems which affects valid usages as well. See the documents in the "External links" section for more information.
Specifically, the authors of prior versions have to be acknowledged and certain "invariant sections" specified by the original author and dealing with his or her relationship to the subject matter may not be changed. If the material is modified, its title has to be changed (unless the prior authors give permission to retain the title). The license also has provisions for the handling of front-cover and back-cover texts of books, as well as for "History", "Acknowledgements", "Dedications" and "Endorsements" sections.
