We consider software licensing to fall into three broad categories:
- copyleft
- permissive or non-reciprocal
- proprietary
We consider as FOSS that software released under a license approved either by the Open Source Initiative or is from, or approved as a free license, by the Free Software Foundation (see also).
Opinions vary widely about what makes a good, or even an acceptable, license choice. One school of thought takes a very live-and-let-live approach, "your code, your license" and considers it bad form to complain about the licenses others choose. Another (eg, the FSF) considers proprietary licensing an ethical hazard the use of which creates an unjust dependency of users on the creators of software.
Copyleft can and will be used with, or even incorporate permissively licensed works (FSF, SFC, SPI)
Some permissive licensing advocates reject proprietary and copyleft works alike (ASF, BSDs)
Proprietary can accept permissively licensed works.
Some permissive licensing advocates reject copyleft but tolerate proprietary incorporation of permissively licensed work.
There's a split within copyleft camp about the importance of pursuiing copyleft enforcement.
Some types of copyleft can conflict: ZFS under CDDL which (at least by some analysis) conflicts with GPL of Linux kernel.
GPLv2 vs GPLv3 (patent clauses, non-compliance remedy)
LGPL
AGPL
[[images/licensing-realms.svg|Licensing adjacencies and barriers]]