Copyfree is not the same as copyleft.
As of this writing, the Free Software Foundation has this to say about copyleft:
Copyleft is a general method for making a program or other work free, and requiring all modified and extended versions of the program to be free as well.
The simplest way to make a program free software is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.
The key to understanding copyleft is in license heritability and the availability of copyrighted material:
Strict copyleft licensing has very strong heritability characteristics. In essence, where document
foois covered by a copyleft license and document
baris covered by some other license, combining the two documents in a single work must, by law, result in a work available under the terms of the copyleft license.
Strict copyleft licensing imposes restrictions on allowable forms of distribution. Specifically, where any part of a copyleft licensed work is distributed and the distributed portion is not in a "preferred" form for editing the work (i.e. "source" form), the source form must be distributed as well if the recipient so desires.
In addition, as a direct result of the above two necessities, a copyleft license also has the following characteristics:
Copyleft licenses are dependent upon copyright law for their core characteristics to remain enforceable. One cannot legally enforce restrictions on redistribution, outside of a direct contractual agreement, without some form of copyright control in effect.
Copyleft licenses are mutually incompatible. Any two copyleft licenses cannot in and of themselves be combined into a single, integrated work because of the conflicting heritability requirements of both, where each license requires not only that the covered work should continue to be distributed under the same terms even if it is modified, but that the other work (previously distributed under a different license) should also, in its entirety, be redistributed under the same terms. In short, each license requires the other license to change.
There are exceptions to this last requirement in some circumstances, but apart from explicit exceptions for particular licenses the applicability of such exceptions is subject to interpretation by the courts. It may thus be legally dangerous to rely on such exceptions.
Contrast With Copyfree
Meanwhile, a copyfree license must conform to the following requirements, as explained in the Copyfree Standard:
- Free Use
- Free Distribution
- Free Modification and Derivation
- Free Combination
- Universal Application
This contrasts sharply with the requirements of copyleft licensing, whose demands contradict at least the Free Distribution and Free Combination requirements of the Copyfree Standard.
Meaning of Share-Alike
The term "share-alike" has gained a lot of recognition thanks to its use in the name of one of the most popular Creative Commons licenses, the Creative Commons Attribution Share-Alike License (or CC-BY-SA). "Share-alike" is essentially synonymous with one characteristic of copyleft licensing: the requirement for derivative works to adopt the same license. Share-alike licenses are subject to the same license compatibility issues described above for copyleft licenses.
There is some question as to the specifics of how the heritability terms of a license must be defined to qualify as a "share-alike" license, because it is established less strongly by precedent than the same characteristics of copyleft licenses such as the GPL. As a rule of thumb, however, share-alike licenses should be regarded as copyleft licenses.
A number of prominent free/open source licenses are copyleft (or "share-alike") licenses. Examples include:
A small set of copyleft licenses rapidly growing in popularity, particularly in Free Culture communities rather than Open Source Software communities, is the set of Creative Commons licenses with Share-Alike conditions -- also known collectively here as CC-SA. These copyleft terms for Creative Commons licenses are offered in two different Creative Commons licenses, providing the CC-BY-SA (Attribution/Share-Alike) and CC-BY-NC-SA (Attribution/Non-Commercial/Share-Alike).
By far the best known and most prominent copyleft license is the GNU General Public License, or GPL. The most recent version of this license is Version 3. In addition to GPLv2's usual copyleft terms and preferential treatment (but not prescriptive requirement) for noncommercial uses, the GPLv3 adds additional restrictions and reinforcements of restrictions it inherited from earlier versions.
The Mozilla Public License, or MPL, is used by one of the most widely known pieces of open source software in the world, the Firefox Web browser.