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Licenses
There are many different public copyright licenses available for use with software, art and other creative works. They allow authors to grant permissions to others for redistribution and modification which would otherwise violate normal copyright. Please note copyright law is separate from patent and trademark law. Those may impose additional restrictions on the use of a work.
No license
Without an explicit license default copyright law usually applies. All rights are reserved and no one may copy, redistribute or make derivatives of the work. However, publishing on some platforms like GitHub and YouTube requires accepting a terms of service which may automatically grant some rights to others. In general, if you want your work to be shared or built upon you should always pick a license. The Free Software Foundation doesn't recommend neglecting to choose a license because it will render the work non-free.
Public domain
Works automatically enter the public domain in most countries after the life of the author plus 70 years. It also consists of things which are not eligible for copyright like math formulas, some government works and logos consisting only of typefaces, words, slogans or simple geometric shapes. There is no easy way for authors to completely donate their work to the public domain before the copyright expires. The easiest way to essentially opt-out of copyright is to use a license that surrenders as many rights as possible like CC0.
Free Software Foundation
These licenses were made by the Free Software Foundation to support the GNU project and other free software.
The General Public License was originally written by Richard Stallman and is probably the most popular free software license. It guarantees end users the four essential freedoms that define free software. Those are the freedoms to run, study, share and modify the software. Essentially, you are free to do what you want with software released under this, provided that if you release it to the public you must also release the source code. Source code does not have to be released if binaries to modifications are not released to the public.
The GPL is copyleft in that any derived work needs to be distributed under the same terms. In this way it demands the work stays free so it can benefit everyone. The work can never be used in proprietary software. Think of it as a license that grants you every freedom, except the freedom to take away other people's freedom over the software. This license highly favors the end users. The kernel, Linux is licensed under the GPLv2.
The Affero GPL is essentially the GPL with an additional provision that users interacting with the software over a network must also get the source. With the popularity of "cloud" services, GPL software can be changed and run at a server without the obligation to release the source code. This is called the "application service provider loophole" and the AGPL closes it. The FSF recommends the AGPL for programs that are likely to be used on servers like interactive web applications, mail, web and game servers.
The Lesser GPL is a free software license that lets people link to the work in their proprietary software under certain conditions. It is usually used for shared libraries.
The GNU Free Documentation License is a copyleft license for free documentation. It is similar to the GPL, giving readers the rights to copy, redistribute, and modify a work and requires all copies and derivatives to be available under the same license, rendering it GPL-incompatible. Copies may also be sold commercially, but, if produced in larger quantities (greater than 100), the original document or source code must be made available to the work's recipient.
The GFDL was designed for manuals, textbooks, other reference and instructional materials, and documentation which often accompanies GNU software. However, it can be used for any text-based work, regardless of subject matter.
Creative Commons
Creative Commons does not recommended their licenses for software. They are more appropriate for articles, blogs, music, photographs and websites. All CC licenses provide the base right to distribute the work for non-commercial purposes without modification. The differences between them are based on four conditions: attributions (BY), share-alike (SA), non-commercial (NC) and no derivatives (ND).
CC0 tries to relinquish the work to the public domain as reliably as possible.
CC BY allows commercial redistribution of the work and derivatives as long as the author is credited.
CC BY-SA is the same as CC BY except derivatives must also be licensed under the same terms. In this way it is also a copyleft license.
CC BY-ND allows commercial redistribution of the unmodified work as long as the author is credited. No derivatives are allowed.
CC BY-NC allows redistribution of the work as long as the author is credited and it is not used commercially.
CC BY-NC-SA only allows redistribution if the author is credited, it is done under the same license terms and it is not used commercially.
CC BY-NC-ND only allows redistribution if the author is credited, no changes are made and it is not used commercially.
Apache
The Apache license is a permissive FOSS license known for protecting users against patent suing. Versions prior to 2.0 are not GPL compatible. The FSF recommends Apache 2.0 over the similar MIT License because of its included patent license. They also recommend it over copyleft licenses for small programs that are 300 lines or shorter.
Massachusetts Institute of Technology
The MIT License is permissive similar to the 3-clause BSD license. It allows the work to be reused in proprietary software. MIT licensed software can also be used in GPL software, but the reverse is not true. People can more or less do whatever they want with the work as long as credit is given and the author isn't held liable. Both MIT and BSD licenses do not have an express patent license, because both were made before software patents were recognized in the US. There are several MIT License versions which are all functionally identical.
Expat License
Expat was an old free software XML parser which used this license. This is the version of the MIT License certified by the Open Source Initiative.
X11 License
This is a modified version of expat used by X11. The only difference from Expat is that it prohibits the X Consortium's name from being used in advertisements without permission. The FSF notes that this version should be called the "X11 License" or the "MIT/X11 Consortium License" to remove ambiguity.
Berkeley Software Distribution
The BSD licenses are a family of permissive licenses. Unlike copyleft licenses, they do not have share-alike terms which require derivative works to use the same license. They were made for the free forks of BSD. There are three types of BSD licenses.
4-clause license (Original BSD License)
The original BSD license contained a clause not found in later licenses, known as the "advertising clause". This clause eventually became controversial, as it required authors of all works deriving from a BSD-licensed work to include an acknowledgment of the original source in all advertising material. Richard Stallman claimed that he saw over 2000 names in BSD in 1996.
3-clause license ("Revised BSD License", "New BSD License", or "Modified BSD License")
In this BSD license, the advertising clause was removed from the license text in the official BSD on 22 July 1999. Other BSD distributions removed the clause, but many similar clauses remain in BSD-derived code from other sources, and unrelated code using a derived license.
This version allows unlimited redistribution for any purpose as long as its copyright notices and the license's disclaimers of warranty are maintained. The license also contains a clause restricting use of the names of contributors for endorsement of a derived work without specific permission.
2-clause license ("Simplified BSD License" or "FreeBSD License")
This license is even more simplified version has come into use, primarily known for its usage in FreeBSD. The primary difference between it and the New BSD (3-clause) License is that it omits the non-endorsement clause. It also adds a further disclaimer about views and opinions expressed in the software.
Other
- ISC License is like BSD/MIT but clearer given that there is one ISC license and 3 BSD licenses and 2 MIT licenses. The license used by OpenBSD.
- WTFPL - DO WHATEVER THE FUCK YOU WANT TO PUBLIC LICENSE. The name says it all.
- Kopimi - While not truely a license, puting the Kopimi mark on your work shows that you intend for the downloader to copy and redistribute the work as much as possible. It is sometimes considered to be the equivalent to CC0 when used by itself, but really it is only a deceleration that you intend for this work to be copied. It can be used in combination with any other license that permits other users to copy, including those with limited rules associated (such as Non-Commercial and Attribution Licenses).
External links
- ChooseALicense -- Github's How to choose a license for your own work
- GNU Project's guide to choose a license for your own work
- TLDRLegal -- Explanation of licenses, in plain English.