Alfred Hitchcock's Psycho is the mother of the slasher genre, birthing a cinematic trend of eerie settings and psychological horror mixed in with a seemingly indestructive killer that has no genuine motive outside of their crippling thoughts and messed up brain.
Psycho follows Marion Crane (Janet Leigh), currently hiding from police and others by taking back roads after stealing $40,000 from her employer with plans of using the money to escape with her boyfriend. She arrives at a desolate Bates Motel to rest during a harsh rainstorm, she meets the hotel's Norman Bates (Anthony Perkins), who appears to be your average polite hotel worker and owner.
The film's psychological elements come into play through Norman, who appears to have an incredibly strange relationship with his mother. Marion is invited to have dinner with Norman, but strangely his mother intervenes. There's not much to really say here without spoiling the plot too much, but the film uses some elements that are often found in more recent slasher films, primarily Friday the 13th.
Psycho isn't primarily there for the abnormally large body-count, though. It's a film that focuses on the narrative and psychological aspect of the killer, rather than the crazy lengths he will go to just wreak havoc among people. It's eerie, unsettling, well-made, and a classic.