The Hogwarts Archives are, presumably, a collection of papers and books written by Hogwarts' Headmasters or Professors. Professor Albus Dumbledore left many papers of his authorship or property in his will to the Archives after his death. Among them could be found a set of notes on The Tales of Beedle the Bard. 
Behind the scenes
- Presuming that they actually exist, the Hogwarts student records may be a part of the Archives, as may the woodcuts and engravings of early Hogwarts students.