The Daily Prophet Newsletters were three-page documents available to members of the Official Harry Potter Fan Club in 1998 and 1999. The UK publishers of the Harry Potter series, Bloomsbury, created the Fan Club and it was only available to British citizens. Four newsletters were printed, and were written by J.K. Rowling herself.
The newsletters were written from an in-universe point of view, detailing aspects of the wizarding world. Some of the information in the newsletters doesn't fit exactly with information from later books, since they were printed before the release of Harry Potter and the Goblet of Fire. Some of the character names and information did find their way into the later books or other merchandise, such as the Famous Wizard Cards.