Though her Untitled Film Stills (1977–80)--a now-iconic series showing Sherman herself in various cinema-inspired guises and settings--seemed familiar, they eluded simple explanation. They were not based on specific films or well-known actresses; cracks in the facades of these self-dramatizations revealed their artificiality, and yet these photos still looked like copies. They amounted to an almost encyclopedic list of female roles in Hollywood films, B-movies, film noir and European auteur cinema of the 1950s and 60s. They represented a challenging commentary on the stereotypical, cinema-derived notions of femininity in viewers’ minds.
What's intriguing/unusual/"groundbreaking" for this work is that it is the first non-photographic work that Sherman has produced (or, at least, made available to the public). Sherman's process for this series is described in the linked essay (by Mark Westall).
(Note, BTW, the final "hammer price" for the Untitled Film Stills series...)