The choosealicense.com site can help you quickly chose a license for your digital content. For more detailed information the following article provides a good overview of licensing and licensing options from the perspective of scientists who write code:
Morin, A., Urban, J., and Sliz, P. “A Quick Guide to Software Licensing for the Scientist-Programmer” PLoS Computational Biology 8(7) (2012): e1002598.
Fundamentally what matters is that there is a clear statement as to what the license is, and that the license is one already vetted and approved by the Open Source Initiative.
An important point to consider is that a license is best chosen from the very start of a project - even if for a repository that is not public. Pushing off the decision only makes it more complicated later, because each time a new collaborator starts contributing, they, too, hold copyright and will thus need to be asked for approval once a license is chosen.
Popular open source licenses
These few licenses are by far the most popular:
The GNU-GPL is different from most other open source licenses in that it is “infective”: anyone who distributes a modified version of the code, or anything that includes GPL’ed code, must make their code freely available as well.
Manuals, reports, manuscripts and other creative works are eligible for intellectual property protection and are hence automatically protected by copyright, just as software source code. Creative Commons has prepared a set of licenses using combinations of four basic restrictions:
- Attribution: derived works must give the original author credit for their work.
- No Derivatives: people may copy the work, but must pass it along unchanged.
- Share Alike: derivative works must license their work under the same terms as the original.
- Noncommercial: free use is allowed, but commercial use is not.
Only the Attribution (CC-BY) and Share-Alike (CC-BY-SA) licenses are considered “Open”.