The fact that an author is anonymous meand that the identity of the autor is unknown, so that it is not disclosed. In other words, it means that the author is not identified by name.
Anonymous works refer to all those works, either of art or literature, that have an anonymous, undisclosed, or unknown creator or author.
For instance, one type of anonymous work, are all those stories which was spread since the were passed from mouth to mouth thorough generations, such as fables.