GNU General Public License
The GNU General Public License (GNU GPL or simply GPL) is a widely used free software licence, originally written by Richard Stallman for the GNU project. The GPL is the most popular and well-known example of the type of strong copyleft licence that requires derived works to be available under the same copyleft. Under this philosophy, the GPL is said to grant the recipients of a computer program the rights of the free software definition and uses copyleft to ensure the freedoms are preserved, even when the work is changed or added to. This is in distinction to permissive free software licences, of which the BSD licences are the standard examples.
The GNU Lesser General Public Licence (LGPL) is a modified, more permissive, version of the GPL, originally intended for some software libraries. There is also a GNU Free Documentation Licence, which was originally intended for use with documentation for GNU software, but has also been adopted for other uses, such as the Wikipedia project.
The Affero General Public Licence (GNU AGPL) is a similar licence with a focus on networking server software. The GNU AGPL is similar to the GNU General Public Licence, except that it additionally covers the use of the software over a computer network, requiring that the complete source code be made available to any network user of the AGPLed work, for example a web application. The Free Software Foundation recommends that this licence is considered for any software that will commonly be run over the network.
FlightGear and GPL
FlightGear is released under the GNU GPL, which means that everyone may edit, change, or use models, source and textures freely. Most planes and scenery are also released under GPL.
Contributors releasing their contributions under the terms of the GPL to the FlightGear project should keep in mind that such contributions are non-revocable, open-source projects in general, work such that GPL'ed contributions are usually not removed once a contributor abandons/leaves the project, be it temporarily or indefinitely. To clarify: It is very common for open source projects that contributors show up for a period of time and then take some hiatus from the project -sometimes even for years, thus it is extremely uncommon for contributions to be removed from the project, despite the fact that we've seen some of our most-talented contributors requesting their contributions be removed after some form public/private community debate or disagreement. However, requesting removal of contributions shows a misunderstanding of the GPL, and of FlightGear in particular: As a project, we cannot afford having contributors who may spend months (or even years) contributing to the project, only to see them pull their contributions once there's even the slightest form of community disagreement. Unfortunately, it is typical to see especially artwork contributors (aircraft, scenery, base package) making such requests - obviously, because the corresponding resources are usually much more self-contained than core contributions (source code).
Thus, please think about all the repercussions of contributing to FlightGear before causing community irritation by asking for your contributions to be removed at some point - what you get and see now when you download and install FlightGear is the result of over a decade of contributions from hundreds of contributors, no matter if it's source code, artwork, aircraft, scenery, documentation, wiki articles or tens of thousands of discussions in the archives.
We cannot afford having contributors who feel that they're entitled to pull their contributions from the project once there's even the slightest bit of community disagreement, no matter if it's removing aircraft, scenery, wiki articles or forum threads - for the sake of the project, please consider your GPL'ed contributions final - otherwise, the project will always be in danger, because some of our key contributors may decide to ask for significant contributions to be removed from the project.
Honestly, it's been causing lots of irritation in the community whenever key contributors have asked for their contributions to be removed - and while there are no legal grounds that justify complying with such requests, we've still been trying to accomodate them, simply for the sake of peace and out of respect for long-time community members.
Imagine just for a second that you were to download FlightGear 3.0, enjoy lots of neat features like advanced weather, Rembrandt or the c172p, but once you download FlightGear 3.2 six months later, you'll notice that it no longer provides support for the very same features, just because some contributor got fed up with the project and asked for his contributions to be removed.
However, that's simply not how the project (and open source in general) works - if you think that you are entitled to ask for your contributions to be removed at some point, please contribute to another project, not FlightGear.