The public domain encompasses all creative works that do not have copyright protections. Works in the public domain can be freely used and adapted without permission. 

Works enter the public domain when their copyright term, or the length of time a work is subject to copyright protection, expires. Some works that failed to comply with registration and notice requirements may also be in the public domain, even if the duration of their copyright term has not yet elapsed. Today, copyright protections are automatic--you do not need to provide a copyright notice or register your copyright--but in the past, copyright holders were required to include a copyright notice on a work and register with the U.S. Copyright Office in order to be eligible for copyright protections.