What is copyleft and why does it matter?
In short: copyleft is the antithesis of copyright.
Copyright claims exclusive ownership/authority over something (a computer program, a work of writing or art, etc.) and restricts if or how others may use it. In modern society it is the norm for almost everything that can be found on computers, where it is often referred to as unfree or proprietary software. Some particularly (in)famous examples include Adobe Photoshop, Microsoft Windows, Mac OS, iTunes, etc.
Copyleft, on the other hand, gives everybody the freedom to use, deconstruct, modify, and redistribute something however they please, with the inherent caveat that they must grant others the same freedoms (reciprocation) if they distribute a derivative version. In the context of computer programs it is often referred to as free open-source software (FOSS) - note that "free" in this context refers to freedom of use, not necessarily to lack of cost.
Copyleft licenses require four essential freedoms to be granted at all times:
The freedom to use the work for any purpose.
The freedom to study the work and modify it.
The freedom to redistribute the work to others.
The freedom to distribute modified versions of the work.
The most famous copyleft license is the GNU General Public License (GPL) for computer software, which is found on examples such as the Linux kernel. However, there are other copyleft licenses available for software, as well as for other types of works entirely, such as Creative Commons Attribution-ShareAlike (CC BY-SA) which is often used for photos/art/etc.
Something important to note is that copyleft is not the same as public domain. Public domain licenses are not copyright, but unlike copyleft, they do not require reciprocation, which can be problematic because it allows someone to turn their modified version into a proprietary/unfree work. One (in)famous example of this is Disney taking old fairytales or legends and modifying them to create copyrighted works which they then jealously guard from "infringement" of any sort. The reciprocation clause in copyleft seeks to prevent that sort of exploitation.
Why does this all matter? Because "intellectual property" is a scourge upon utility and art alike. Copyright laws ensure that corporate monopolies, and all the degradations that come alongside them, are practically inevitable. Here is a video covering some specific examples in more detail. I am against the concept of intellectual property and therefore I believe copyleft should be the norm.
I make a point of using FOSS whenever possible and I recommend you should too - not only for moral reasons, but also because it's often safer and more reliable than proprietary software. The source code being publicly available allows people to check that there is not something bad like spyware lurking within; even if you don't know how to parse coding yourself, usually others have already checked it. Additionally, a work being copyleft often provides you with more alternatives in the form of "forks", which may have improved features or updates compared to the original. Check out sites like opensourcesoftwaredirectory.com as a starting point for examples!
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