Copyleft comes from the idea of copyright. When the last phrase refers to the rights that a perosn has over something he/she has "invented" then copyleft is about giving the right to modify the work of someone else in order that the same is allowed for others as well.
Bit it's not so easy with copyleft, there are different levels in it. Sometimes some parts of the work can be modified an sometimes all.
1. Weak copyleft case is for example Mozilla Public License. With weak copyleft license person can make modifications to the work code and add additional settings to the code. But the person should not use parts of the code and only link to the original licensed code.
The most important thing in weak copyleft is that source code copied or changed under the MPL must stay under the MPL (Wikipedia, 2010)
With weak copyleft strong copyleft can't be used, as these criterias doesn't fit the main license rules.
Mozilla Public license is defined at Mozilla nonprofit organization page. Some examples of the softwares under MPL 1.1 license are: Bugzilla, Tinderbox, Bonsai.
Bugzilla is a server software designed to help manage software development. The soft itself helps to find bugs in developed software and handle server issues. At the same time the Bugzilla itself is developed by international volunteers who want to participate in making the version better and better to contribute their needs.
Weak copyleft
2. Strong copyleft case - GPL and GNU General Public License.
With strong copyleft license a person can incorporate code from projects licensed under the permissive license, or strong copyleft license under which software is licensed. It basically means that by using some code parts from GPL code, perosn ahs to agree and disribute hes/her own work under the same GPL license.
While using strong copyleft it's unlikely that someone else will use your code as a source of their work. Strong copyleft might end up in several legal issues.
A comment at 451 CAOS Theory, On the fall and rise of the GNU GPL syas that the strong copyleft is probably on the fall and weak one is on the rise. Especially because week copyleft gives a choice
3. No copyleft case - X11 license, Apache license and the BSD licenses.
This means that someone's work can be sued without the need to give something back to the community. No copyleft is supported by those who would like to become free from the legal restrictions and enjoy and use development made by different communities and people. With no copyleft one doesn't have to worry about any legal issues.
No copyleft should make it quicker and easier to use best solutions for everyone's work. At the same time it allows to distribute the outcome free or commercially. But the creators of original software creators still have to be credited ;)
Sources:
1. Copyleft law Tracking and analysying the key legal developments of the Internet Age. Licensing 101 for open source projects - Pick a license, part II.
2. Mozilla Public Linense Version 1.1 , 25. March
3. Wikipedia. Mozilla Public License , 26. march 2010
4. 451 CAOS Theory, On the fall and rise of the GNU GPL, 25. March 2010
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