Putting software and other material in a public place is not the same as making it publicly usable. In order to do that, the authors must also add a license, since copyright laws in some jurisdictions require people to treat anything that isn’t explicitly open as being proprietary.

While dozens of open licenses have been created, the two most widely used are the GNU Public License (GPL) and the MIT/BSD family of licenses. Of these, the MIT/BSD-style licenses put the fewest requirements on re-use, and thereby make it easier for people to integrate your software into their project.

For an excellent short discussion of these issues, and links to more information, see Jake Vanderplas’s blog post from March 2014 at astrobetter.com/blog/2014/03/10/the-whys-and-hows-of-licensing-scientific-code. For a more in-depth discussion of the legal implications of different licenses, see Morin et al., 2012 \cite{22844236}.