Copyleft is a general concept; to copyleft an actual program, you need to use a specific set of distribution terms. There are many possible ways to write copyleft distribution terms, so in principle there can be many copyleft free software licenses. However, in actual practice nearly all copylefted software uses the GNU General Public License. Two different copyleft licenses are usually “incompatible,” which means it is illegal to merge the code using one license with the code using the other license; therefore, it is good for the community if people use a single copyleft license.
Noncopylefted Free Software
Noncopylefted free software comes from the author with permission to redistribute and modify, and also to add additional restrictions to it.
If a program is free but not copylefted, then some copies or modified versions may not be free at all. A software company can compile the program, with or without modifications, and distribute the executable file as a proprietary software product.
The X Window System illustrates this. The X Consortium releases X11 with distribution terms that make it noncopylefted free software. If you wish, you can get a copy which has those distribution terms and is free. However, there are nonfree versions as well, and there are (or at least were) popular workstations and PC graphics boards for which nonfree versions are the only ones that work. If you are using this hardware, X11 is not free software for you. The developers of X11 even made X11 nonfree for a while; they were able to do this because others had contributed their code under the same noncopyleft license.
Lax Permissive Licensed Software
Lax permissive licenses include the X11 license and the two BSD licenses. These licenses permit almost any use of the code, including distributing proprietary binaries with or without changing the source code.
GPL-Covered Software
The GNU GPL (General Public License) is one specific set of distribution terms for copylefting a program. The GNU Project uses it as the distribution terms for most GNU software.
To equate free software with GPL-covered software is therefore an error.
The GNU Operating System
The GNU operating system is the Unix-like operating system, which is entirely free software, that we in the GNU Project have developed since 1984.
A Unix-like operating system consists of many programs. The GNU system includes all the GNU software, as well as many other packages, such as the X Window System and TeX, which are not GNU software.
The first test release of the complete GNU system was in 1996. This includes the GNU Hurd, our kernel, developed since 1990. In 2001 the GNU system (including the GNU Hurd) began working fairly reliably, but the Hurd still lacks some important features, so it is not widely used. Meanwhile, the GNU/Linux system, an offshoot of the GNU operating system which uses Linux as the kernel instead of the GNU Hurd, has been a great success since the 90s.
Since the purpose of GNU is to be free, every single component in the GNU operating system has to be free software. They don’t all have to be copylefted, however; any kind of free software is legally suitable to include if it helps meet technical goals. And it isn’t necessary for all the components to be GNU software, individually. GNU can and does include noncopylefted free software such as the X Window System that were developed by other projects.
GNU Programs
“GNU programs” is equivalent to GNU software. A program Foo is a GNU program if it is GNU software. We also sometimes say it is a “GNU package.”
GNU Software
GNU software is software that is released under the auspices of the GNU Project. If a program is GNU software, we also say that it is a GNU program or a GNU package. The README or manual of a GNU package should say it is one; also, the Free Software Directory identifies all GNU packages.
Most GNU software is copylefted, but not all; however, all GNU software must be free software.
Some GNU software was written by staff of the Free Software Foundation, but most GNU software comes from many volunteers. (Some of these volunteers are paid by companies or universities, but they are volunteers for us.) Some contributed software is copyrighted by the Free Software Foundation; some is copyrighted by the contributors who wrote it.
Nonfree Software
Nonfree software is any software that is not free. Its use, redistribution or modification is prohibited, or requires you to ask for permission, or is restricted so much that you effectively can’t do it freely.
Proprietary Software
Proprietary software is another name for nonfree software. In the past we subdivided nonfree software into “semifree software,” which could be modified and redistributed noncommercially, and “proprietary software,” which could not be. But we have dropped that distinction and now use “proprietary software” as synonymous with nonfree software.
The Free Software Foundation follows the rule that we cannot install any proprietary program on our computers except temporarily for the specific purpose of writing a free replacement for that very program. Aside from that, we feel there is no possible excuse for installing a proprietary program.
For example, we felt justified in installing Unix on our computer in the 1980s, because we were using it to write a free replacement for Unix. Nowadays, since free operating systems are available, the excuse is no longer
