Free software is software that comes with permission for anyone to use, copy, and/or distribute, either verbatim or with modifications, either gratis or for a fee. In particular, this means that source code must be available. "If it's not source, it's not software." This is a simplified description; see also the full definition.
If a program is free, then it can potentially be included in a free operating system such as GNU, or free versions of the GNU/Linux system.
There are many different ways to make a program free—many questions of detail, which could be decided in more than one way and still make the program free. Some of the possible variations are described below. For information on specific free software licenses, see the license list page.
Free software is a matter of freedom, not price. But proprietary software companies typically use the term "free software" to refer to price. Sometimes they mean that you can obtain a binary copy at no charge; sometimes they mean that a copy is bundled with a computer that you are buying, and the price includes both. Either way, it has nothing to do with what we mean by free software in the GNU project.
Because of this potential confusion, when a software company says its product is free software, always check the actual distribution terms to see whether users really have all the freedoms that free software implies. Sometimes it really is free software; sometimes it isn't.
Many languages have two separate words for "free" as in freedom and "free" as in zero price. For example, French has "libre" and "gratuit". Not so English; there is a word "gratis" that refers unambiguously to price, but no common adjective that refers unambiguously to freedom. So if you are speaking another language, we suggest you translate "free" into your language to make it clearer. See our list of translations of the term "free software" into various other languages.
Free software is often more reliable than nonfree software.
Open source software
The term "open source" software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licenses that we consider too restrictive, and there are free software licenses they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free.
We prefer the term "free software" because it refers to freedom—something that the term "open source" does not do.
Public domain software
Public domain software is software that is not copyrighted. If the source code is in the public domain, that is a special case of noncopylefted free software, which means that some copies or modified versions may not be free at all.
In some cases, an executable program can be in the public domain but the source code is not available. This is not free software, because free software requires accessibility of source code. Meanwhile, most free software is not in the public domain; it is copyrighted, and the copyright holders have legally given permission for everyone to use it in freedom, using a free software license.
Sometimes people use the term "public domain" in a loose fashion to mean "free" or "available gratis." However, "public domain" is a legal term and means, precisely, "not copyrighted". For clarity, we recommend using "public domain" for that meaning only, and using other terms to convey the other meanings.
Under the Berne Convention, which most countries have signed, anything written down is automatically copyrighted. This includes programs. Therefore, if you want a program you have written to be in the public domain, you must take some legal steps to disclaim the copyright on it; otherwise, the program is copyrighted.
Copylefted software is free software whose distribution terms ensure that all copies of all versions carry more or less the same distribution terms. This means, for instance, that copyleft licenses generally disallow others to add additional requirements to the software (though a limited set of safe added requirements can be allowed) and require making source code available. This shields the program, and its modified versions, from some of the common ways of making a program proprietary.
Some copyleft licenses, such as GPL version 3, block other means of turning software proprietary, such as tivoization.
In the GNU Project, we copyleft almost all the software we write, because our goal is to give every user the freedoms implied by the term "free software." See our copyleft article for more explanation of how copyleft works and why we use it.
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.
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" 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 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 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 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 applicable; we do not use any nonfree operating systems, and any new computer we install must run a completely free operating system.
We don't insist that users of GNU, or contributors to GNU, have to live by this rule. It is a rule we made for ourselves. But we hope you will follow it too, for your freedom's sake.
The term "freeware" has no clear accepted definition, but it is commonly used for packages which permit redistribution but not modification (and their source code is not available). These packages are not free software, so please don't use "freeware" to refer to free software.
Shareware is software which comes with permission for people to redistribute copies, but says that anyone who continues to use a copy is required to pay a license fee.
Shareware is not free software, or even semifree. There are two reasons it is not:
- For most shareware, source code is not available; thus, you cannot modify the program at all.
- Shareware does not come with permission to make a copy and install it without paying a license fee, not even for individuals engaging in nonprofit activity. (In practice, people often disregard the distribution terms and do this anyway, but the terms don't permit it.)
Private or custom software is software developed for one user (typically an organization or company). That user keeps it and uses it, and does not release it to the public either as source code or as binaries.
A private program is free software in a trivial sense if its sole user has full rights to it.
In general we do not believe it is wrong to develop a program and not release it. There are occasions when a program is so useful that withholding it from release is treating humanity badly. However, most programs are not that important, so not releasing them is not particularly harmful. Thus, there is no conflict between the development of private or custom software and the principles of the free software movement.
Nearly all employment for programmers is in development of custom software; therefore most programming jobs are, or could be, done in a way compatible with the free software movement.
Commercial software is software being developed by a business which aims to make money from the use of the software. "Commercial" and "proprietary" are not the same thing! Most commercial software is proprietary, but there is commercial free software, and there is noncommercial nonfree software.
For example, GNU Ada is developed by a company. It is always distributed under the terms of the GNU GPL, and every copy is free software; but its developers sell support contracts. When their salesmen speak to prospective customers, sometimes the customers say, "We would feel safer with a commercial compiler." The salesmen reply, "GNU Ada is a commercial compiler; it happens to be free software."
For the GNU Project, the emphasis is in the other order: the important thing is that GNU Ada is free software; whether it is commercial is just a detail. However, the additional development of GNU Ada that results from its being commercial is definitely beneficial.
Please help spread the awareness that free commercial software is possible. You can do this by making an effort not to say "commercial" when you mean "proprietary."