Some people love Freud. Some people hate the guy. Some people see Freud as a scientist. Some folks see him as a storyteller.
No matter which way you slice 'im, Freud is recognized as the godfather of psychopathology, or the study of mental illness, psychosis, abnormal behavior, and mental development (it's a real grab bag). Film, books, even human behavior itself was fundamentally changed by Freud's proclamation that the surface is just the beginning.
Freud believed you have to peel back the proverbial onion to root out the real motivations. And in that belief, he radically changed the very way we think about babyhood and personalities, fantasies and blabbermouths, blunders and obsessions, sexuality and repression. You might know him from the concept of the Freudian slip, which is one of those awkward moments where you call your boyfriend "Daddy" by accident (whoops!). For Freud-lovers, that moment reflects your unconscious or repressed desires, finally being spoken aloud (in the most embarrassing way possible).
While the Freudian slip may be his most enduringly popular legacy, Freud broke onto the scene with his study of dreams, which he referred to as the "royal road to our unconscious." Then he really made a name for himself studying all manner of mental illnesses with folks who agreed to plop down on his couch and spill their oh-so-repressed guts.
Although Freud cranked out some of greatest hits when it came to psychoanalytic terms, like transference (falling in love with your therapist), sublimation (putting all your anger into boxing instead of smashing your Pop in the face), and anal stage (you figure that one out yourself), he also had a bunch of patients, to whom he gave wacky pseudonyms like Dora, Rat Man, Wolf Man, and Little Hans.
Despite all these psychoanalytical achievements, the enduring impact of Freud's work has been on literary criticism, art history, and feminism, more than the therapist's couch. To this day, you are much more likely to find Freud being discussed in a literature class on Shakespeare (that Hamlet really had a thing for his mother) than you are to find him being referenced in a class on clinical psychology. These days, his theories don't hold much water, and if you really want to undergo classic, Freudian psychoanalysis, you'll have to look far and wide for a doctor who offers that anymore. Oh, and you have to go five days a week.