When a work is no longer restricted under copyright law it goes into the public domain. This happens naturally when copyright runs out but with CC licenses a work can also be specifically dedicated to the public domain.
In the public domain there are no restrictions for use - anyone can do anything they want with it. Examples of public domain works are the writings of Edgar Allen Poe or the musical scores of Chopin. Specific recordings of Chopin's music however maybe copyrighted by the musician(s) recording them.
The public domain mark is used to identify a work which has naturally moved into the public domain because its copyright has run out. The length of copyright coverage can be confusing because the length is based upon the life of the author plus a variable number of years (depending on when it was created.) This mark, when applied to works known to be out of copyright, helps all users to know when a work can be freely used.
CC Zero Tool
The CC Zero tool is not a license, but is an option which waives all the copyrights of the creator to the marked work. It dedicates the work to the public domain so that not even attribution of the creator is required. It may be applied to works that are still in copyright and can only be applied by the copyright holder.