About the GPL

My old WhiteLove theme is licensed under the GPL. So what is the GPL? Put simply, you can remove the author’s name, but you must keep the code open source. Users are allowed to modify it however they like, but you have to follow the corresponding terms. The main principles of the GPL:

It ensures the software is always released as open source from start to finish, protecting the work from being stolen and sold commercially. Any piece of software that uses the source code of third-party software protected by the GPL, and is distributed to people outside the development team, automatically becomes an entity protected and bound by the GPL itself. In other words, at that point it must open its source code.
The GPL is essentially an expression of copyleft (also translated as “anti-copyright,” “copyright left,” “all rights reversed,” and so on). You can remove all of the original work’s copyright information, as long as you keep it open source and include the GPL license with the source code and binaries, so that those who come after can clearly see how the software is licensed. The essence of the GPL is that, as long as the software stays fully open source, users get as much room as possible to do what they want with it, so the software develops faster and better.
No matter how the software is distributed, the source code must be included. For example, if it’s offered for download on the web, a link to download the source code must be clearly provided on the same page as the binary download (if there is one). If it’s distributed on disc, a disc with the source files must be included.
Companies or individuals who develop or maintain software under the GPL may charge users a certain service fee. But the same old rule applies: the complete source code of the software must be provided free of charge, and the source code may not be bundled with services or sold in any disguised bundle. This doesn’t apply to themes that don’t declare that they follow the GPL. Of course, all my work follows this license. WhiteLove and the Vivian theme I’m using now were both modified from existing theme frameworks.

Wikipedia says:

Rights granted: The terms and conditions of the GPL apply to anyone who receives a work under the GPL (the “licensee”). Any licensee who accepts these terms and conditions is granted permission to modify, copy and redistribute the work or derivative versions of it. The licensee may charge a fee for this service, or not. This is the biggest difference between the GPL and other free software licenses that prohibit commercial use. Stallman believes free software should not restrict commercial use, and the GPL states this clearly. But the GPL also stipulates that distributors may not restrict the rights the GPL grants. For example, this prohibits distributing the software under a shrink-wrap (passive, implied) license or contract. Distributors under the GPL also agree that patents used in the software may be used in other GPL software. Copyleft: The GPL does not grant licensees unlimited rights. The right to redistribute is granted only if the licensee makes the software’s source code, and all modifications, available. Copies and modified versions must also be licensed under the GPL. These requirements are what copyleft is, and it rests on the fact that the work is copyrighted. Because it is copyrighted, the licensee has no right to modify or redistribute it (except under fair use) unless there is a copyleft clause. Anyone who wants to exercise rights normally prohibited by law only needs to agree to the terms of the GPL. Conversely, if someone distributes software in violation of the GPL (for example, by not releasing the source code), they may be sued by the original author. Copyleft uses copyright law to achieve the opposite of its purpose: copyleft gives people inalienable rights rather than the many restrictions imposed by copyright law. This is why the GPL is called “hacked copyright law.” Many distributors of GPL software bundle the source code with the executables. Another approach is to provide the source code on physical media (such as a CD). In practice, a lot of GPL software is distributed over the internet, and much of the source code is also available via FTP. Copyleft only takes effect when a program is redistributed. Modifications to the software don’t have to be made public or open source, as long as they aren’t distributed. Note that copyleft applies only to the software, not to the software’s output (unless the output is the software itself). This may change in version 3 of the GPL, however. The GPL is a license: The GPL is designed as a license, not a contract. In common-law countries there is a clear legal distinction between a license and a contract: a contract is enforced by contract law, while the GPL, as a license, is enforced by copyright law. In many civil-law countries, however, there is no such distinction. The principle of the GPL is simple: under copyright law, if you don’t follow the GPL’s terms and conditions, you don’t have the corresponding rights. And without the GPL, copyright law applies to the work by default; the work does not enter the public domain.

For details, see GNU General Public License