Free and Open Source Software (here referred to as F/OSS for "free, libre open source software") describes a concept of declaring or licensing a creative work to encourage and ensure continued access, enhancement, and re-use by and for all. This position has been popularly codified into the Creative Commons group of licenses, but originates in the GNU GPL. Further, there are what you could call Churches of Free Software.
The wide dissemination of and dependence on software by civilization supports a very important idea that Code is Law--it "dictates" from below; as such, it is important that software remains free (in the Greek sense of libre to all those whose livelihood and future depend on it.
Until the perfect system is realized, a totalitarian system can never be sustainable in perpetuity. Hence, societies have fought towards democracy (to elect those who rule from above) and why software must evolve toward openness (which, by virtue of its copiability lends itself to pervasive use and rules broadly or laterally. Copyleft sets itself apart from the destructive tendencies of proprietary software by preserving fair attribution, but requiring the software to be open and any modifications intended for public use to be offered likewise; that is, to share-alike. Copyleft does not require (or even encourage) that contributors receive no compensation for their efforts (a common misconception because of the dual meaning of the word "free"), but that a work for distribution to the public remain free for others to continue to update, inspect, repair, share, and enhance. Note that if a system of compensation were in place to fairly reward contributors to a project, the advocates of copyleft would likely be delighted as it would encourage and feed continued community involvement and enhancement.
Free and open source software must be "viral" in order to inhibit the spawning of processes that would otherwise eventually seek to dominate (thereby destroying) the community-sphere. Without the "share and share-alike" clause, Will and secrecy dominate rather than the global community, OR (even worse) few participate because there are no guarantees that their efforts will be preserved for them (generally finding themselves back in a propriety system -- freedom is something that must be upheld lest all default back to the ground state of willful protectionism). As Richard_Stallman points out, it is important not to confuse Power with Freedom. Although power grants a certain kind of freedom, in a global society it can only offer by itself, at best, a stand-off, failing to uphold true freedom. There will always be at least one other who will stand their ground to avoid exploitation. As such, the simple exercise and pursuit of power is a dead-end, much better is to see that both achieve more with cooperation and dialog.
Note also that to say that copyleft's "share-alike" clause reduces one's freedom is like saying that the U.S. Constitution reduces one's freedom. Of course, it is true in a way, but only in a very particular way: to ensure continued freedom for all.
To appreciate the error in the question "What's wrong with proprietary software? Can't people just use something else if they don't like it?" one should look at world history and compare with "What's wrong with totalitarianism? Get out of the country if you don't like our system!". While one may have such "freedom" in theory, in practice, such systems tend toward dependencies which are difficult to climb out of and, in any case, stifle creativity and innovation in the long run. For more exposition on these subjects, see the GNU Project Philosophy, particularly Why Software Should Be Free.
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