A product in the public domain can be shared without restriction. Works in the public domain include:
Most work produced by the United States Government
Works that have an expired copyright (i.e. 70 years after the creator's death)
Works created for the public domain (the creator gives others free reign to reuse or mix content using a Creative Commons license or puts it in the public domain)
An idea expressed in a work or a common fact
Most work produced by the United States government