Straight up: I've never heard of the term 'copyleft' before. The fact that I get that little red squiggle under it tells me it's not really a word. Concisely, my question is, what does it mean to you? I saw it tagged on a few questions so I read the tag description:For questions about the copyleft concept (also known as Share-Alike), a concept that promotes or enforces the use of the same or compatible license for derived works.And I read the Wikipedia page on it. My understanding is that copyleft has attributes of Share-Alike but the author must disclose any other terms alongside it. To me, it seems like a bit of a useless term. Why describe it as copyleft if you need to add a licence to it anyway? Would you not be better off describing it as copyright with already-agreed-upon licenses? Otherwise it's only another word for Share-Alike. 
What is copyleft?
copyleft;terminology
Copyleft is actually a term coined by Richard Stallman (also called RMS) who is the pioneer of the Free Software Movement which ultimately resulted in today's world where people take using FOSS software so much for granted.Stallman originally used this term to distinguish his way of free software licensing (GNU GPL/LGPL) from the other Copyright licenses which are typically designed by proprietary companies to keep the holding rights of a software to themselves by not disclosing the source.You see, freedom to users is the major objective of FSF movement and it doesn't have anything to do with zero cost or free as in beer, it refers to free as in freedom, which is more expressed by the term libre than free.The other camp in the FOSS world, by contrast, prefers to use the term permissive instead of copyleft. This camp favors software licenses like MIT, Apache, BSD, etc. which generally grant all usage rights along with the code. This second camp also prefers the term open source to free software. The reason is that software innovation resulting from source code being openly developed is a far more important objective for this camp than some liberal ideology.Regardless, both these camps continue to contribute their efforts in the FOSS world today and to an extent, respect each other's views.EDITTo elaborate a bit on the legal implications, whilst both permissive and copyleft licenses grant their users freedom along with the code, there is a difference between these two kinds of freedom. Permissive licenses grant freedom to do just about anything with the source code to that one particular user. Copyleft licenses by contrast, tries to secure freedom to it's entire userbase by restricting that particular user's ability to make said source code proprietary or closed. It achieves this by mandating that any further modification to the copyleft licensed source should be distributed on a similar copyleft license terms. This mandate usually gives the incorrect impression to the initial observer that copyleft licenses are restrictive or viral in nature, but a detailed analysis reveals that its not true. Copyleft just tries to secure maximum freedom for the entire community or the user base, not just one user.As I described in one of the comments below, the difference between permissive and copyleft is similar to the one between dictatorship and democracy. Rulers in both kinds of rule usually have the freedom to do anything, but democracy has some restrictions on the ruler since it tries to secure freedom to the entire masses, not just that one dictator.EDIT-2From the above analogy, I don't want to imply any negativity about the word dictator here, nor do I want to send a message that I'm in favor of copyleft licenses and against the permissive ones. In fact, dictatorship itself is a neutral word in Political Science, but only due to actions of a few bad rulers, its usage has acquired a negative connotation. Dictators can be good too (like BDFL Linus Torvalds). Similarly, permissive licensed software, in the right hands could bring out the best for everyone.However, I do think GPL is a good thing because it prevents the appleization of software, but Apple positively surprised us today by taking a strong stance in favor of its users. Tomorrow, it might even go ahead and open-source the OSX stack (though I don't think it that likely in the near future).