My last post brought a new question:
What are the key differences between GNU GPL Free software licenses and public domain software?
Public domain software allows anyone to do whatever they want with software. In short software is not subject to copyright. No person or other legal entity can establish or maintain proprietary interests within a particular legal jurisdiction. See wikipedia article for more information.
GNU GPL is a copyright license. You can use and distribute software under the terms and condition of its license only.
Hope this clears the difference between GPL and public domain software.