As I understand it, any extensions / plugins that you write for the community edition of Alfresco are subject to the terms of the GPL; in other words, if you distribute your application with those changes, e.g. you sell it, then you must also make the source code etc for those changes / extensions available.
Basically, the source code to any application that makes use of code released under a reciprocal license, such as the GPL, must be made available; the GPL essentially "infects" the application. A caveat to this is that you are not obliged to make your changes available IF the application is only used within your organization or is for personal use; in other words, if you don't distribute it.
Other licenses such as the Mozilla license and the LGPL are examples of partially closable licenses meaning they do allow you to write closed source plug-ins etc - if you make any changes to the core product then you must release those changes also. For example, you could write a closed source Firefox extension but you couldn't modify Firefox and distribute it without releasing your changes.
Those 15 lines of source code you want to keep closed source must be really good
Disclaimer: I'm not a lawyer, this is just my understanding of the various licenses. You have been warned
Basically, the source code to any application that makes use of code released under a reciprocal license, such as the GPL, must be made available; the GPL essentially "infects" the application. A caveat to this is that you are not obliged to make your changes available IF the application is only used within your organization or is for personal use; in other words, if you don't distribute it.
Other licenses such as the Mozilla license and the LGPL are examples of partially closable licenses meaning they do allow you to write closed source plug-ins etc - if you make any changes to the core product then you must release those changes also. For example, you could write a closed source Firefox extension but you couldn't modify Firefox and distribute it without releasing your changes.
Those 15 lines of source code you want to keep closed source must be really good
Disclaimer: I'm not a lawyer, this is just my understanding of the various licenses. You have been warned
So if understand this correctly, I can sell my extension, as long as I provide source.
Yes; that is my understanding. You must include the source code with the binary distribution. Take a look at the FAQ - I've linked to the section on writing plug-ins but might be a good idea to read the rest of it as well: http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins
Also, the stand-alone app can remain closed, right?
As long as your application is a completely separate application - a different process - doesn't use any of the Alfresco stuff and all you are doing is calling the web services API of an Alfresco repository, then as I understand it you can license it how you please. There is the concept of aggregation in the GPL (http://www.gnu.org/copyleft/gpl-faq.html#MereAggregation) meaning that you can distribute both GPL and non-GPL licensed code and the programs are treated as separate works with separate copyrights; it basically comes down to how the programs communicate, e.g. shared memory, inter-process communication etc.
Having said all of that you will still probably want to get all of this checked by a lawyer
