About public domain works
The public domain is a growing body of works that are available for people to re-use with few restrictions. Many galleries, libraries, archives and museums (GLAM) add material to the public domain, as well as individual creators. Works can enter the public domain in a number of ways, for example the copyright term may have expired, or the creator may decide to dedicate the work to the public domain or the work was never entitled to copyright protection in the first place (in the case of facts and ideas). There are two public domain tools:
The public domain mark indicates works which are free of copyright (for example because the copyright term has expired). It is simply a label and is not legally enforceable but it exists to help users discover works that are free of copyright.
CC Zero is very different and indicates that a creator has dedicated their work to the public domain and waived their copyright (i.e. before copyright protection has expired). These works can be reproduced freely. Although no attribution is required, in all cases it is good practice to give credit to the creator where you can. See CC0 waiver.