Copyleft as opposed to copyright used to reserve all rights so as to distribute and use software free and also would be the same for modified versions of it
Copyleft has three variations: Strong, weak, and absent:
Strong copyleft preserve exactly the same license for all the derivative works. It means that any modifications of a software with copyleft license would have the same conditions and totally free to use, distribute, copy, and change. This variant of copyleft would limit the most the production of proprietary software from the free one. GNU General Public License was the first copyleft license. For the first time GNU used this term that nobody can limit the freedom of users by applying any restrictions.
Weak copyleft license does not allow that all the copyleft rights transmit to derivates of copylefted software. LGPL or GNU Library General Public License was given to the GNU C library. The strategy for this program and also the similar library ones is to use LGPL license to allow it to compile with a proprietary program. In this strategy, both proprietary and free software developers have the equal right to take advantage of library programs.
Absent copyleft is used for the kind that derived program is not free that has BSD licenses. BSD license allows proprietary activities and compilation with proprietary software. Apache license is a good example of a BSD alike. For instance, Apache allows be compiling, using and distributing in both free and proprietary programs. Stronghold is a software created by C2Net that combines Apache with a commercial software. Stronghold is sold by Red Hat and is commercially licensed.
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