Copyleft is a play on the word copyright, for the practice of using copyright law to give freedom to the users of your work, on the condition that the same rights and freedoms be preserved in modified versions of the work.
Copyleft is found in licenses such as CC-BY-SA, the SA or "ShareAlike" indicating the the condition of continued sharing. This is the license used by Appropedia. Other examples include GFDL and computer software licenses.
 Applying copyleft
Common practice for using copyleft is to codify the copying terms in a license. Any such license typically gives each person possessing a copy of the work the "Four Freedoms" (from the Free Software Definition):
- 0. the freedom to use the work,
- 1. the freedom to study the work,
- 2. the freedom to copy and share the work with others,
- 3. the freedom to modify the work, and the freedom to distribute modified and therefore derivative works.