Here is a summary from the GNU website:
After a year and a half of public consultation, thousands of comments, and four drafts, version 3 of the GNU General Public License (GPLv3) was finally published on June 29. While there's been a lot of discussion about the license since the first draft appeared, not many people have talked about the benefits that it provides developers. We've published this guide to fill that gap. We'll start with a brief refresher on free software, copyleft, and the goals of the GPL. We'll then review the major changes in the license to see how they advance those goals and benefit developers.
The Foundations of the GPL
Nobody should be restricted by the software they use. There are four freedoms that every user should have:
- the freedom to use the software for any purpose,
- the freedom to change the software to suit your needs,
- the freedom to share the software with your friends and neighbors, and
- the freedom to share the changes you make.