Free Software, Free Society: selected essays of Richard M. Stallman. 2nd edition.

This diagram, originally by Chao-Kuei and updated by several others since, explains the different categories of software. It’s available at http://gnu.org/philosophy/categories.html as a Scalable Vector Graphic and as an XFig document, under the terms of any of the GNU GPL v2 or later, the GNU FDL v1.2 or later, or the Creative Commons Attribution-Share Alike v2.0 or later. To view a copy of the Creative Commons license, visit http://creativecommons.org/licenses/by-sa/2.0, or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California 94105, USA.

Free Software

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, at http://gnu.org/licenses/license-list.html.

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.