Authors have the right to decide how their works should be used, distributed or modified.
The new technologies mean two very important changes on creative process:
1.- New ways to create: digital photography, 3D modeling programs, audio and video digitalization, publication of articles through blogs on the Internet, design or photography retouch programs; are just some examples of the techniques which bring new ways of creativity making it more accessible to people.
2.- Ease to communicate creative works through Internet: Allowing authors to distribute the work not only in their countries, but in the whole world without the need of the service of other professionals and companies. They can do it with lowest costs.
So we have a lot of people being able to produce creative works, more ways to create and it is much easier to distribute them. This changes have open new copyright dimensions: the open licenses and between them the copyleft ones.
Until a few decades ago, to achieve authors rights to be a reality, so to say, to enforce third parties to use the work as the author requested, there was one copyright model in the sense of "all rights reserved"; which means that in order to modify, distribute, publish or copy a work, anybody had to ask the author to get permission case by case. This way of copyright was the only way to guarantee authors what to do with their works.
Once the digital format appeared to support works, and the ease of communication and diffusion the Internet brings, some authors saw the possibility to put their works online, but not to leave them untouchable by default, but instead to allow others to add parts, improve, sell, distribute, modify them with derivatives having to bear the same possibilities or others...
Open software was pioneer in this new dimension of copyright: copyleft is a way to license in such way authors not only do not prohibit some uses of their works, but want others being able to make derivatives and commercial uses of the those without having to request specific permission as long as the derivatives allowed the same uses too.
Some authors want, for instance, that their work might be distributed, copied, reproduced or modified without having to get specific permission. Others allow distribution or reproduction as long as it is without commercial purposes. In this case they reserve for themselves only some economic rights for those commercial purposes. Same might happen with derivatives. Some allow them, others don't.
Generally speaking these are the open licenses being some of them copyleft.
In the last years, the most used protection used for contents called "all rights reserved", also called "full copyright" has been complemented with the "some rights reserved" philosophy including copyleft licenses.