FreeCAD's license is the GNU LGPL , and it's recommended that workbenches also be licensed under the LGPL if they are to be distributed in the FreeCAD source code.  This page provides info to clarify distinctions between the GNU LGPL license and the normal GNU GPL license, as well as provide some general information on GNU licensing.
Difference between LGPL and GPL
Key distinction between LGPL and GPL: "LGPL permits use of the library in proprietary programs; using the ordinary GPL for a library makes it available only for free programs." 
“There are reasons that can make it better to use the Lesser GPL in certain cases. The most common case is when a free library's features are readily available for proprietary software through other libraries. In that case, the library cannot give free software any particular advantage, so it is better to use the Lesser GPL for that library.” 
Related to Creative Commons licenses
A note on "Copyleft”
"It is a legal mistake to use a backwards C in a circle instead of a copyright symbol. Copyleft is based legally on copyright, so the work should have a copyright notice. A copyright notice requires either the copyright symbol (a C in a circle) or the word 'Copyright'.
A backwards C in a circle has no special legal significance, so it doesn't make a copyright notice."