Computer software can be put into categories based on common function, type, or field of use. There are three broad classifications:
The GNU Project categorizes software by copyright status: free software, open source software, public domain software, copylefted software, noncopylefted free software, lax permissive licensed software, GPL-covered software, the GNU operating system, GNU programs, GNU software, FSF-copyrighted GNU software, nonfree software, proprietary software, freeware, shareware, private software and commercial software.
Free software is software that comes with permission for anyone to use, copy and 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." If a program is free, then it can potentially be included in a free operating system such as GNU, or free versions of the Linux system.
Free software in the sense of copyright license (and the GNU project) is a matter of freedom, not price. But proprietary software companies typically use the term "free software" to refer to price. Sometimes this means a binary copy can be obtained at no charge; sometimes this means a copy is bundled with a computer for sale at no additional charge.
Open source software
Open source software is software with its source code made available under a certain license to its licensees. It can be used and disseminated at any point, the source code is open and can be modified as required. The one condition with this type of software is that when changes are made users should make these changes known to others. One of the key characteristics of open source software is that it is the shared intellectual property of all developers and users. The Linux operating system is one of the best known examples of a collection of open source software.
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 block other means of turning software proprietary.
Copyleft is a general concept. Copylefting an actual program requires a specific set of distribution terms. Different copyleft licenses are usually “incompatible” due to varying terms, which makes it illegal to merge the code using one license with the code using the other license. If two pieces of software use the same license, they are generally mergeable.
Non-copylefted free software
Noncopylefted free software comes from the author with permission to redistribute and modify and to add license restrictions.
If a program is free but not copylefted, then some copies or modified versions may not be free. 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 approach. 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, nonfree versions are available and workstations and PC graphics boards for which nonfree versions are the only ones that work. The developers of X11 made X11 nonfree for a while; they were able to do this because others had contributed their code under the same noncopyleft license.
Shareware is software that comes with permission to redistribute copies, but says that anyone who continues to use a copy is required to pay. Shareware is not free software, or even semi-free. For most shareware, source code is not available; thus, the program cannot be modified. Shareware does not come with permission to make a copy and install it without paying a license fee, including for nonprofit activity.
Like shareware, freeware is software available for download and distribution without any initial payment. Freeware never has an associated fee. Things like minor program updates and small games are commonly distributed as freeware. Though freeware is cost free, it is copyrighted, so other people can't market the software as their own.