Copyleft, its forms and choosing a license

Copyleft

First of all, what exactly is copyleft? The name comes from a pun on the phrase "Copyright – all rights reserved", turning it into "Copyleft – all rights reversed". Copyleft was born due to the ageing licensing laws of copyright, which were largely hard and impractical to use on software, especially the open-source kind. It is distinct from copyright mostly in the sense that it is generally much more lenient in terms of distribution and modification. Of course, different variations exist, but this difference remains.

Variations

Strong copyleft

Strong copyleft licenses are characterized by the demand that all derivations from the original licensed work retain the license of the original. This means that nothing derived from a strong copyleft-licensed creation can be made proprietary. Integrating such modifications into hybrid systems with other kinds of software is much more difficult under a strong copyleft license, however, it is the most beneficial for end-users. Publishing the source code of the creation is also required.

The most common strong copyleft licenses are:

• GNU GPL
• Affero GPL, which is generally similar to the GNU GPL; but additionally deals with the SaaS loophole.

A strong copyleft license may be chosen if the creator does not want anyone deriving their proprietary software from their creation. 

Weak copyleft

A weak copyleft license, like the strong license, requires that any modifications of the original work retain the license. However, some exceptions are set to make software integration easier. This is especially useful in creating code libraries. Like the strong copyleft, the weak copyleft requires the publishing of source code.

Examples of weak copyleft licenses:

• GNU LGPL
• EUPL, can be considered a "variable" license
• MPL

A weak copyleft license would be a wise choice if the software is meant for further development and integration.

Non-copyleft/permissive license

A permissive license is the most lenient of the three, not requiring any sort of license inheritance. It does also not enforce publishing the source code. This sort of license gives the most freedom to the developers.

Examples of permissive licenses:

• BSD license
• MIT license

This sort of license may be good for some component-level programs such as drivers.

Conclusion

The copyleft licenses greatly help in regulating the world of open-source software. There is a multitude of licenses to choose from, at least one of which will be fit for whatever project a developer may have.