User: Jgay/Copyright-license-note
Copyright is a legal concept. It refers to laws that grant individuals or organizations exclusive rights on the use and distribution of a work. Most kinds of creative or practical works (text, music, etc) are covered by copyright law, including software. What copyright law covers is is how you actually express something (e.g., what you write on paper or a digital file), it doesn’t cover the underlying ideas or concepts in a given work. When you make an original work, copyright laws automatically grants you copyright on that work. You don't need to explicitly say you hold copyright or anything, although it is useful to put a copyright notice on works to let people know you have a copyright claim on them.
A copyright notice might look like this: "Copyright © 2016 Joshua Gay". If I hold copyright, then I the law says that I exclusively have the right to do various things with that work, such as make copies (i.e. "copy"-right"), make derivatives, perform the work publicly, sell copies of the work, etc. The law says I have the freedom to do all those things, but it says that you do not necessarily have the freedom to do those things with my work -- if you want to do many of those things, then you will need to get permission from me in order to do so. The law says that the copyright holder has the exclusive right to grant such permission.
If I hold copyright on some work like source code, I will put a notice saying I hold copyright and then I will explicitly let you know exactly what you are allowed to do with the work and what conditions I want you to follow. With source code, I will let you know that you are free to use, study, share, modify, and share modifications of that source code. I may say that you can only do so under certain conditions, for example, I might say that if you share a binary copy of the program, that you must offer people a version of the program in source code form.
Now, as copyright holder, I can put different terms and conditions on each copy I distribute. I can write those conditions however I want to. But, that can get confusing. If a person needs to read a different set of conditions on each and every bit of source code they receive, it can quickly become burdensome. That is why it is useful to just take some general set of terms and conditions that is well understood. For example, the GNU project wrote a set of terms and conditions they call the GNU General Public License (GPL). We wrote it so that anybody who claims copyright can, instead of writing out in detail all the terms and conditions in their own way, simply take a copy of the GPL that the terms and condition on using and sharing the work can be found in the GPL. In a copyright notice, I might say something like: "you are free to use this work under the terms of the GNU GPL version 3.0 as published by the Free Software Foundation, or, at your option, any later version". There are other things I might include in my notice a well. But, usually what I put in a given copyright notice is pretty standard and well understood.
One reason it is helpful to choose a standard license is that it makes it easy for multiple different copyright holders to all put the same set of terms and conditions on the work.
The terms and conditions of the GPL do not require that if you modify the work (e.g., add and/or subtract new code to it), that the copyright holder has to also put their code under the GPL. It does say that whatever terms and conditions you put on the parts you claim copyright on -- that is the license you apply to your contributions -- do not impose any additional restrictions beyond the terms of the GPL. For example, I couldn't have some code that in which my terms were "You can not sell copies of this program" and distribute my code combined with some GPL-licensed code that someone else wrote, because doing so would mean the work I am distributing "violates" the GPL. It violates the terms of hit GPL, because the GPL says you are free to sell copies of the work, and so not allowing sales of the work is an "additional restriction."
This is where the concept of "license compatibility" comes in. The terms and conditions of the code I hold copyright on needs to be "compatible" with the terms and conditions of the code other people hold copyright on if I want to combine the code in some way to form a new work and then distribute that combined version of the work. Licenses like the GPL let you modify the work however you want for your own personal use, but it says when you distribute the modified work, you need to license the work as a whole under the terms of the GPL and the portions you claim copyright on (or the various portions others people claim copyright on ) all need to have licenses that are compatible with the GPL. Separately of course, the various portions of code that are not licensed under the GPL (and are not considered "derivative works" of the GPL-portions of the work) can still be distributed under their own terms -- because, remember, copyright law says the copyright holders retain exclusive rights to choose the terms and conditions. term need
