The Free Software Foundation (FSF) is strongly focussed on championing the cause of free software; they have formulated a specific definition for the term (see Free software definition).
To many encountering it, the term does not imply the FSF's definition of "software that gives users freedom". The experience of most users with software that requires a fee to purchase and use implies that "software that can be had for no fee" is the correct definition. Within the software industry, this latter class of software is usually termed freeware (or a related category, shareware).
One of the most widely-known free software licenses is the GNU General Public License, a copyleft license created by the FSF and under which it releases most of its software (as part of the [GNU project]? to build a complete operating system from free software). Another is the [BSD-style license]?, so called because it is applied to much of the software distributed with the BSD operating systems. Many other licenses are listed at the FSF website (see below).
Another distinction to draw is between free software and software released to the public domain. While both kinds of software allow all the freedoms listed earlier, public domain software has no copyright owner at all, and anyone is able to take the software, claim ownership of it, modify it, and restrict its use, in effect causing that version to become non-free with or without the consent of the original author. With free software, copyright in the work is still held, and it cannot be reassigned or redistributed without the specific license of the copyright holder.
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 BIND? name server, the Sendmail? mail transport server, the Apache web server, the Mozilla web browser, and the Perl, Python, Tcl and PHP programming languages. Like all free software, these projects distribute their programs under licenses that grant users all the freedoms discussed above.
Open Source is a concept closely related to free software. A group of people who went on to form the Open Source Initiative (OSI) coined the term to attempt to avoid the ambiguity of the English word "free", and to bring a higher profile to the practical benefits of 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 freely by developers; this causes a pragmatic appreciation for free software licenses independent from ideological concerns.
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 distinction is subtle, but the FSF considers it significant enough to distance itself from the Open Source term (claiming that free software is the morally correct way to produce software, regardless of whether it produces technically superior software). In most cases, though, licenses which qualify as free software licenses also qualify as open source licenses, and vice versa, so often the two terms are used interchangeably and usually the same people are happy to work on the software regardless of the ideologies involved.
See also:
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