- Putting software under the GPL does not mean abandoning ownership?
Absolutely not. In nations signatory to the Berne Convention, copyright is the default, which means by default only the author (or current copyright holder, if the original author transfers the copyright) may reproduce the work, prepare derivative works, etc.
The author of a work may use a license to permit other people to use the work in specific ways, subject to author-defined license-specific rules. Allowing other people to use the work via a license in no way lessens the author's copyright -- on the contrary, the author's copyright is what makes the license possible and necessary.
It is important to note that any recipient of a GPL-licensed work can distribute and modify it. Thus, even if the author stops distributing a GPL-licensed work, the author cannot rescind the rights previously granted to existing recipients. (This claim is somewhat debated for GPLv2, and may vary by jurisdiction, but is made explicit in GPLv3.) In that sense, releasing a work under the GPL means abandoning the ability to restrict its distribution and modification under the terms of the GPL, but this is tremendously different from abandoning ownership.
- What constitutes ownership? Is it the copyright?
Yes, copyright is a kind of "ownership," in the metaphorical mapping of intellectual property rights onto physical property rights. That is, the copyright holder of a work has certain exclusive rights to a work. Those rights can be granted to other people in limited ways (or unlimited ways, if you're feeling generous), but ultimately the grant of those rights comes from the legal reality that those rights fundamentally belong to the copyright holder and are licensed as the copyright holder pleases.
- The owner is not bound by the GPL, e.g. he could change or improve the code without having to share the changes back to the community?
Absolutely. The copyright holder is never beholden to the rules of the holder's own license grant! The copyright holder's rights stem from copyright law, not from a license grant. The copyright holder can legally prepare a derivative of her own work based on her rights under copyright.
Other people can prepare derivative works of someone else's copyrighted work only insofar as that right has licensed to them. In the case of the GPL, that means that others' derivative works must be GPL-licensed. The copyright holder need not follow this rule, since they they do not derive their rights from the GPL license grant. Rather, they are the original source of the rights being licensed.
- What about community contributions? Whom do they belong to? If not clearly defined, what options do exist?
Contributions are creative works created by their respective authors, and the author of a contribution holds the copyright to that contribution. If a project includes copyrighted work from multiple contributors, then licensing the project requires consent from all copyright holders. In professional open-source project, this is usually handled at contribution-time by making the contributor agree to either
- a contributor licensing agreement (CLA) that grants the project owner the unlimited (or sometimes limited) right to re-license the contribution in the future to another license
- a copyright transfer agreement (CTA) that ensures the project only has one copyright holder, by causing the contributor to transfer the copyright of her contribution to the project owner
If the project includes work held by another copyright holder who refuses to re-license and has not previously agreed to a re-licensing agreement, the project may choose to remove or rewrite the portions written by other people.
If someone takes your GPL-licensed project and distributes a fork with their own changes without interacting with you (which they can do as long as their changes are under a GPL-compatible license), then you are free to incorporate their changes in your own project. However, if you do so, then you are no longer the sole author of the work, and you must abide by the other author's license as long as you use the material they wrote.