Aspects of Licenses

Although there are many different free software licenses available, in the important respects they all say the same things: that anyone can modify the code, that anyone can redistribute it both in original and modified form, and that the copyright holders and authors provide no warranties whatsoever (avoiding liability is especially important given that people might run modified versions without even knowing it). The differences between licences boil down to a few oft-recurring issues:

compatibility with proprietary licenses

The permissive (non-copyleft) free licenses allow the covered code to be used in proprietary programs. This does not affect the licensing terms of the proprietary program: it is still as proprietary as ever, it just happens to contain some code from a non-proprietary source. The Apache License, X Consortium License, BSD-style license, and the MIT-style license are all examples of proprietary-compatible licenses.

compatibility with other types of free licenses

Most of the commonly-used permissive free licenses are compatible with each other, meaning that code under one license can be combined with code under another, and the result distributed under either license without violating the terms of the other. Some of them are also compatible with some of the copyleft licenses, meaning that a work comprised of code under the permissive license and code under the copyleft license can be distributed as a combined work under the copyleft license (since that's the license that places more conditions), with the original code in each case remaining under its original license. Typically these compatibility issues come up between some permissive license and the GNU General Public License (or its variant the the section called “The GPL and License Compatibility”). This topic is discussed in more detail in the section called “The GPL and License Compatibility”.

enforcement of crediting

Some free licenses stipulate that any use of the covered code be accompanied by a notice, whose placement and display is usually specified, giving credit to the authors or copyright holders of the code. These licenses are often still proprietary-compatible: they do not necessarily demand that the derivative work be free, merely that credit be given to the free code.

protection of trademark

A variant of credit enforcement. Trademark-protecting licenses specify that the name of the original software (or its copyright holders, or their institution, etc.) may not be used to identify derivative works, at least not without prior written permission. This restriction can be implemented purely via trademark law anyway, whether or not it is also stipulated by the copyright license, so such clauses can be somewhat legally redundant — in effect, they amplify a trademark infringement into a copyright infringement as well.

Although credit enforcement insists that a certain name be used, and trademark protection insists that it not be used, they are both expressions of the same concept: that the original code's reputation be preserved, and not tarnished by association.

patent snapback

Certain licenses (e.g., the GNU General Public License version 3, the Apache License version 2, the Mozilla Public License 2.0, and a few others) contain language designed to prevent people from using patent law to take away the rights granted under copyright law by the licenses. They require contributors to grant patent licenses along with their contribution, covering any patents licenseable by the contributor that would be infringed by their contribution (or by the incorporation of their contribution into the work as a whole). Then they go further: if someone using software under the license initiates patent litigation against another party, claiming that the covered work infringes, the initiator automatically loses all the patent grants otherwise provided for that work by the license, and in the case of the GPL-3.0 loses their right to distribute under the license altogether.

Most of these stipulations are not mutually exclusive, and some licenses include several. The common thread among them is that they place demands on the recipient in exchange for the recipient's right to use and/or redistribute the code.