Software programs, like other creative works, are released to its users under certain terms and conditions called licenses. When a license gives its users the rights/freedoms to use, study, copy, modify, improve, and redistribute it, then the software is considered free, or libre, and open-source software (FOSS).
Background: In software development, companies and developers write software as a collection of many files called the source code or the code base. When the software is ready for use, they compile the source code into executable files. For example, applications on Windows and macOS typically have the file extensions .exe and .app, respectively. These executable files are usually unreadable and recovering the source from them is usually impossible.
- If the developers keep their source code private, then the software is said to be proprietary or closed-source.
- If the developers publish the source code for the public to study it, but do not grant them the all freedoms of open-source software, then software is called source-available.
In conventional software development, companies release proprietary software and they require you to purchase a license or subscription in order to use it. This sometimes works well for consumers, but there are important restrictions to be aware of when using proprietary software. If a software program prevents you from exporting your data and using it in another compatible program, then you are forced to maintain a subscription for it in order to maintain access to your work. This tactic, known in economics as vender lock-in or consumer lock-in, is a way for technology companies to make it difficult for you to stop using their services.
Free and open-source software avoids locking-in consumers and instead provides them with several valuable rights:
- Users of free and open-source software are permitted to use it for any purpose (except for those prohibited by law).
- Users and third-parties can independently study and inspect FOSS programs to verify the authenticity of claims regarding its privacy and security. By making the software transparent, it has the potential to be safer and more trustworthy.
- After obtaining copies of the source code, users can modify it to fit their needs. These modifications may include improvements on the original code or removals of existing features (e.g., those that invade privacy, create security vulnerabilities, or are simply unnecessary).
- Users of FOSS can choose to redistribute their software, modified or not, to other people without fee or for profit. The right to redistribution allows users to share their modifications and improvements with others.
Some FOSS licenses require that any redistribution of the software must also be licensed with the same license as the original software or at least be licensed in a way that does not revoke any of the rights granted by the original license. These licenses are known as copyleft licenses and are meant to guarantee that any modifications of FOSS remain part of the community as FOSS.
Example: The strongest copyleft license for FOSS is considered to be GNU Affero General Public License Version 3.0, or AGPLv3, because it requires that anyone who uses the software to provide a service over a network must also provide its complete source code, even if it's modified.
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