GFDL is the GNU Free Documentation License, the copyright under which Wikipedia formerly published.

Wikipedia articles:

• Merge and delete – Why merging and the deleting the original page is normally not permitted under GFDL
• Wikipedia copyrights
• Copyright disclaimers on images
• What the GFDL is not

Use of Wikipedia content in other works:

• Reusing Wikipedia content
• Verbatim copying under the GFDL

Full text:

• Full text of the GNU Free Documentation License