The term free software is used in essentially two different ways:
These definitions may conflict and a piece of software that is free in the first sense may not be free in the second, and vice versa.
Free software of the "free speech" type is sometimes called "software libre", from the French "logiciel libre" and the Spanish "software libre". In fact, in many languages there isn't this conflict between free as in freedom and free as in "free beer": "libre" translates to "free" in the sense of "freedom". Free software of the other type is called "gratis", which translates to the "free" of "free beer".
Various types of free software in this sense exists:
Shareware is not a type of free software, since its license requires payment for use beyond a specified trial period. The payment typically has to be made by the user on an "honor system". Using shareware beyond the trial period without payment is a (common) breach of copyright law. Warez, software which is distributed for free by a third party in violation of its copyright license, is also not considered to be free software in this sense.
The freedom definition of "free software" has been championed by the Free Software Foundation (FSF) of notable software author Richard Stallman, who codified his philosophy of software freedom in the 1980s.
The FSF has produced a specific free software definition (http://www.gnu.org/philosophy/free-sw); a software is "free" in this sense if it grants:
A list of compliant licenses is available from FSF's web site (see below). The term "proprietary software" is used for software distributed under more traditional licenses which don't grant these freedoms. Usually, copyright law reserves most rights of modification, duplication and redistribution for the copyright owner; software released under a free software license specifically rescinds most of these reserved rights.
The FSF definition of free software does not touch on the issue of price; a commonly used slogan is "free as in speech, not as in beer", and it is common to see CDs of free software such as Linux distributions for sale. However, in this situation the buyer of the CD would have the right to copy and distribute his CD. The FSF definition of free software in fact can conflict with the free beer definition of software. Many free beer software applications forbid the end user from commercially profiting from the software or otherwise charging for the software. This conflicts with the right to redistribute.
To avoid confusion, some people use the words "libre" and "gratis" to avoid the ambiguity of the English word "free". However, these alternative terms are still used mostly within the free software movement and are only slowly spreading to the outside world. Others advocate the term open source software but the relationship between this term of free software is complex and will be more fully explained below.
There are many variations on free software:
See free software licenses for more information.
Note that the original copyright owner of copyleft-licensed software can also make a modified version under their original copyright, and sell it under any license they like, in addition to distributing the original version as free software. This technique has been used as a business model by a number of free software companies; this does not restrict any of the rights granted to the users of the copyleft version.
A large, and ever-growing, amount of software is made available under free software licenses; observers of this trend (and adherents to it) often refer to this phenomenon as the free software movement. Notable free software projects include the Linux and BSD operating system kernels, the GCC compiler, GDB debugger and C libraries, the BIND name server, the Sendmail mail transport server, the Apache web server, the MySQL and PostgreSQL relational database systems, the Perl, Python, Tcl and PHP programming languages, the X Window System, the GNOME and KDE desktop environments, the OpenOffice office suite, the Mozilla web browser and the gimp graphics editor.
Like all free software, these projects distribute their programs under licenses that grant users all the freedoms discussed above, but because of technicalities in the licenses, combining programs by mixing source code or directly linking binaries may be problematic unless both applications are under mutually compatible licenses.
However, when programs are not directly linked together into a single program, these problems do not exist. Much free software can run on non-free platforms such as Microsoft Windows, and non-free software can be run on free platforms, although purists prefer to use all-free software running on a free platform such as Linux. Free software packages constitute a software ecosystem[?] where different pieces of software can provide services to one another, leading to co-evolution of features: in one simple example, the Python programming language provides support for the HTTP protocol, and the Apache web server that provides the HTTP protocol can call the Python programming language to serve dynamic content.
Open Source movement, which is philosophically distinct from the free software movement, was created by a group of people who formed the Open Source Initiative (OSI). They sought to bring a higher profile to the practical benefits of sharing software source code and to interest industry and major software houses in the concept. These advocates see the term open source as avoiding the ambiguity of the English word "free" in free software. Many people recognise a qualitative benefit to the software development process when a program's source code can be used, modified and redistributed by developers.
Since the OSI places emphasis on the pragmatic benefits of access to the program's source code, rather than focusing on user and programmer freedoms, the FSF has distanced itself both from the Open Source movement and from the term "Open Source". The free software movement places primary emphasis on the moral or ethical aspects of software, seeing technical excellence as a desirable by-product of its ethical standard. The Open Source movement sees technical excellence as the primary goal, regarding source code sharing as a means to an end.
In all cases, licenses which qualify as free software licenses also qualify as open source licenses. However, the reverse is a different matter since the Open Source Definition (OSD) (http://opensource.org/docs/definition) does not explicitly and unambiguously state a requirement that open source licenses grant people the right to copy their software. For example, nowhere in the OSD or its rationale is the word "copy" included. Rather, some interpret the OSD as treating software like cars which you can inspect, tinker, modify and even resell ("redistribute"), while making copies is a different matter which the OSD never addresses. Note, however, that many interpret the term "redistribution" as used in the OSD to include copying.
If the OSD is treated as a distribution scheme, as Richard Stallman holds, then the right to copy software is necessarily implied by any OSD license. This view is strengthened by statements made by backers of the OSD that the term "open source software" is simply a "marketing campaign for Free Software". However, the proliferation of use licenses (notably by Microsoft) has led many people to believe that a license is required to run software. From that perspective, the OSD (by itself) does not grant nor imply any right to copy software unless the term "redistribute" is interpreted as including the act of copying.
Since the OSI only approves free software licenses as complying with the OSD, most people interpret it as a distribution scheme, and freely interchange 'open source' with 'free software'. And even though there are important philosophical differences between the two terms, particularly in terms of the motivations for developing and using such software, they seldom make any impact in the collaboration process.