GNU General Public License
The GNU General Public Licence (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.