The book had Mary Crane from Dallas, Texas as the leading lady. Since a real Mary Crane exists, Alfred Hitchcock changed her into Marion Crane from Phoenix, Arizona. The movie starred Janet Leigh, Anthony Perkins, John Gavin, Vera Miles, and Simon Oakland.
Note: Wikipedia contains spoilers in its discussions of films, novels, etc.
The first scene in (risque in its day) shows Marion Crane (played by Leigh) and her boyfriend Sam Loomis (played by Gavin) in their undies after a lunchtime "quickie". Crane returns to work and receives $40,000 in cash from her boss to deposit at the bank. Instead of depositing the money she leaves town with it with the intention of marrying her boyfriend. Just out of town she buys a different car because she believes she is being followed; on the way to her boyfriend's she misses a turnoff and eventually ends up on a nearly-deserted road. This road was originally the main route, so it has an old motel on it. She stops in at the Bates motel, run by Norman Bates (played by Perkins) because it is raining and she keeps drowsing off.
Although it's seldom used, Bates keeps the motel open to divert him from taking care of his ill mother.
It turns out that Bates' mother is not ill physically, but mentally. She slashes Marion to bits in the famous shower scene (with its now trademark score by Bernard Herrmann, featuring the screeching violins). Unlike Mary from the novel, Marion is not decapitated in the scene. Bates is horrified when he finds the corpse, but cleans up as if he has done this several times before.
The plot of the rest of the film deals with the search for Crane. Marion's sister, Lila Crane (played by Vera Miles), and boyfriend, Sam, hire a private detective, Milton Arbogast (played by Martin Balsam), to find her. He traces her to Bates' hotel and eventually meets the same fate as Marion. Lila and Sam next go to the motel to follow up when the private detective disappears.
At the end of the film the audience learns that Bates' mother is really dead and that Bates periodically assumes her personality; the dominant half of his personality is his re-imagining of his mother. The Bates personality has no idea that his mother is dead, so has no knowledge of "her" crimes. The last scene shows Bates totally taken over by his "mother."
Psycho was the first film to introduce a single main character and then kill her halfway into the film--a rather shocking turn of events in 1960, with no apparent indication of where the story might go afterwards.
Although there is technically no visible gore portrayed on the screen, the cinematic effect and masterful directing manage to keep the infamous "shower scene" in a very short list of truly frightening and spell binding milestones of cinema.
Psycho is consistently in the top 25 on the Internet Movie Database's list of top 250 films, was #18 on American Film Institute's 100 Years, 100 Movies and #1 on its 100 Years, 100 Thrills, and has been deemed "culturally significant" by the United States Library of Congress and selected for preservation in the National Film Registry. The film spawned four sequels (Psycho II,Psycho III, Psycho IV and Bates Motel - the last a TV movie). The orginal was remade nearly shot for shot, but in color, in 1998 by Gus Van Sant.