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There are many different types of licenses software can be released under. This page gives an overview of the most common ones you may encounter.
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 FOSS 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 code, 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 Linux kernel is licensed under GPLv2.
The Affero GPL is essentially the GPL with an additional provision that users accessing 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. The FSF recommends the AGPL is used for any software that will often be run over a network.
The Lesser GPL is a free software license that lets people use or link the work in their proprietary software under certain conditions. It is usually used for shared libraries.
All CC licenses provide the base right to distribute the work for non-commercial purposes without modification. The difference between them results from combinations of four other conditions: attributions (BY), share-alike (SA), non-commercial (NC) and no derivatives (ND).
CC BY allows commercial redistribution of the work and derivatives as long as the original 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. It is better suited to art than software.
Berkeley Software Distribution
The BSD licenses are permissive. They contrast with copyleft licenses in that they do not have share-alike terms which require derivative works to use the same license. You basically can do anything you want. You can even incorporate BSD-licensed code in your closed-source proprietary software, effectively re-licensing the source code, so long as you give the original author(s) credit as appropriate.
The Apache licenses are permissive free software licenses known for protecting users against patent suing.
- MIT License is similar to BSD - permissive, and doesn't hold you liable.
- ISC License is like BSD/MIT but clearer given that there is one ISC license and like 6 BSD/MIT licenses, and that no one knows what the fuck an Expat is. The license used by OpenBSD.
- WTFPL - DO WHATEVER THE FUCK YOU WANT TO PUBLIC LICENSE. The name says it all.