The GPL and LGPL (for libraries and API’s) are the two GNU copyleft licenses. In both cases, the source code must be available either as a bundle shipped directly with the project or linked to via a URL in the project documentation. Free software does not refer to price, but instead it is a matter of liberty.

For the GPL (GNU General Public License), all derivative works are also GPL’ed. Also, any project that uses any GPL’ed code will be GPL’ed as well.

The LGPL (GNU Lesser General Public License) is used in cases where you want your project to remain open sourced, but you do not want to force people utilizing your code to GPL their code too. For example, if I released a LGPL’ed JCE framework; developers could use my library in their project without being required to LGPL or GPL their project. A real world example of code licensed under the LGPL is GNU’s own glibc libraries. This allows proprietary software vendors (i.e. Oracle, Corel etc…) to sell code for Linux distros even though it was linked with the glibc libraries. Note that fixes/updates to an LGPL’ed project are considered LGPL’ed.

GNU recommends using the GPL. They have a discussion as to why you should not use the LGPL.

I am appalled that people put out code under GPL or LGPL. By doing so they are giving full permission to child pornographers or militaries fighting illegal wars to use their software without restriction for even the most heinous purposes. To me, these people are like the plumbers who kept Auschwitz running.