Free content refers to works, creations, or information without special restrictions that people can freely use, copy, modify, and redistribute. It is a concept that extends the concept of freedom to use free software freely to all works. In other words, it is called a non-free work.
Free works include works in the public domain for which copyright has expired, as well as works that have copyright but are allowed to be used freely according to the above standards.
Free Works License
A free work may be a copyleft that must be distributed under a license with the same restrictions after modifications have been made to the work, or the modified version may be distributed under different terms (sometimes not as a free work).
The GNU Free Documentation License is an example of a copyleft license, and the BSD License is an example of a non-copyleft license.
In the case of works distributed under a Creative Commons license, if there are no restrictions on “non-profit” or “no change”, they are free works. However, it does not belong to a free work if there is a non-commercial or anti-modification restriction.
GNU Free Documentation License
(English) Definition of "free cultural work" - Widely used as a definition of public works.
(Korean) Definition of "free cultural work" translated from above Archived July 22, 2011 - Wayback Machine