Richard Stallman wrote the GPL. He tried to make sure that everybody could modify, study, use, and change software
To do this he wrote a license that states that if you modify GPL software and
This essentially guarantees that the user of your software has the same rights you have.
When you hear the term free think ``free as in speech'' (libre, frei) rather than ``free as in beer'' (gratis, kostenlos).
Open Source software is an attempt to make the GPL more ``business friendly''. It basically requires you to make the source code available. It is a weaker (in a requirements sense) license than the GPL.