Copyleft

We are living in the world where everything has its price and everything has its rights. What can be done with one thing, can't be done with another. To restrict the rights of services or products or etc.. society uses laws and restrictions - one of them is Copyright. Copyright is a set of rights which are granted to the author or creator of an original work. It includes the right to copy, distribute and adapt, so the author has full control over his/her work. For example intellectual property, patents, trademarks..
The opposite of copyright is copyleft which is also known as ShareAlike in creative commons. It sounds like game with words and it is.
Copyleft's goal is to use copyright to remove the rights of piece of work that come by default from the law. To remove the restrictions to copy, distribute or adapt it. But it has its catch - the same rights has to have the work which source has copyleft rights which means that all derived works must be distributed under the same terms as the original work.
It can be said that copyright can be strong, weak or neither of them.
Weak copyleft licenses are used to create for example software libraries, like Mozilla Public License. Weak means that only some of the derivative work needs to be distributed under a copyleft license.
Strong copyleft forbids linking the subject work into larger works or projects that are not licensed with a compatible copyleft terms. For example GPL.