The Open-source License Landscape is Changing

There's no such thing as "the" open-source license. There are lots of them. Sixty-nine to be precise if one accepts the Open Source Initiative (OSI) as the definitive arbiter of what is open source and what is not.

Some are essentially legacy licenses; in general, the continued proliferation of licenses has abated in recent years but it's often more trouble than it's worth to fully retire licenses that are still in use by active software. Others won't be relevant to a specific type of copyrighted material, such as software programs. (Material under an open-source license is still copyrighted; indeed, copyright law is integral to the working of open-source licensing.)

However, when it comes to open-source software licenses specifically, there are two broad categories. One includes "copyleft" licenses, of which the General Public License (GPL) is the best known. The other includes "permissive" licenses, most notably the Apache, BSD, and MIT licenses.

Different licenses impose more, fewer, or different types of restrictions within that general framework. But those two categories capture the core philosophical distinction. A copyleft license requires that if changes are made to a program's code, and the changed program is distributed outside an organization, the source code containing the changes must likewise be distributed. Permissive licenses don't.