What Is Copyleft?
The simplest way to make a program free is to put it in the public domain (18k characters), uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software (18k characters). They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.
In the GNU project, our aim is to give all users the freedom to redistribute and change GNU software. If middlemen could strip off the freedom, we might have many users, but those users would not have freedom. So instead of putting GNU software in the public domain, we ``copyleft'' it. Copyleft says that anyone who redistributes the software, with or without changes, must pass along the freedom to further copy and change it. Copyleft guarantees that every user has freedom.
Copyleft also provides an incentive for other programmers to add to free software. Important free programs such as the GNU C++ compiler exist only because of this.
Copyleft also helps programmers who want to contribute improvements to free software get permission to do that. These programmers often work for companies or universities that would do almost anything to get more money. A programmer may want to contribute her changes to the community, but her employer may want to turn the changes into a proprietary software product.
When we explain to the employer that it is illegal to distribute the improved version except as free software, the employer usually decides to release it as free software rather than throw it away.
To copyleft a program, first we copyright it; then we add distribution terms, which are a legal instrument that gives everyone the rights to use, modify, and redistribute the program's code or any program derived from it but only if the distribution terms are unchanged. Thus, the code and the freedoms become legally inseparable.
Proprietary software developers use copyright to take away the users' freedom; we use copyright to guarantee their freedom. That's why we reverse the name, changing ``copyright'' into ``copyleft.''
Copyleft is a general concept; there are many ways to fill in the details. In the GNU Project, the specific distribution terms that we use are contained in the GNU General Public License (available in HTML, text, and Texinfo format). The GNU General Public License is often called the GNU GPL for short.
An alternate form of copyleft, the GNU Lesser General Public License (LGPL) (available in HTML, text, and Texinfo format), applies to a few (but not all) GNU libraries. This license was formerly called the Library GPL, but we changed the name, because the old name encouraged to use this license more often than it should be used. For an explanation of why this change was necessary, read the article Why you shouldn't use the Library GPL for your next library.
The GNU Library General Public License (available in HTML and text format) is still available though it has officially been replaced by the Lesser GPL.
The GNU Free Documentation License (FDL) (available in HTML, text and Texinfo) is a form of copyleft intended for use on a manual, textbook or other document to assure everyone the effective freedom to copy and redistribute it, with or without modifications, either commercially or noncommercially.
The appropriate license is included in many manuals and in each GNU source code distribution.
The GNU GPL is designed so that you can easily apply it to your own program if you are the copyright holder. You don't have to modify the GNU GPL to do this, just add notices to your program which refer properly to the GNU GPL.
If you would like to copyleft your program with the GNU GPL, please see the instructions at the end (20k characters) of the GPL text. If you would like to copyleft your program with the GNU LGPL, please see the instructions at the end (27k characters) of the LGPL text.
Using the same distribution terms for many different programs makes it easy to copy code between various different programs. Since they all have the same distribution terms, there is no need to think about whether the terms are compatible. The Lesser GPL includes a provision that lets you alter the distribution terms to the ordinary GPL, so that you can copy code into another program covered by the GPL.