Crime, Exploitation, Spaghetti Western
Quentin Tarantino has been called a cinematic kleptomaniac. He borrows constantly from all genres because he loves all genres. A true video geek from an early age, he'd watched just about everything, and his legendary stint as a video store clerk gave him a deep appreciation for the genre film.
As it often is in the classic "pulp" genre, Pulp Fiction is first and foremost a crime movie. Tarantino said he started out with the cliché crime stories everyone's seen a million times: the mob boss' sexy wife who you don't touch on pain of death; the boxer who's supposed to throw a fight and doesn't do it; the hit men who rain lead on everyone; the crime couple on the run. (Source)
But Tarantino subverts the crime genre. He makes it comical; he has hitmen who just won't shut up; his couple on the run are idiots; the boss' wife isn't at all what you'd expect; his boxer finds redemption; the mob boss gets into the ultimate non-cool situation.
Tarantino always had a fondness for exploitation films. These films have been called the "delicious, forbidden sleazy underbelly of cinema" (source). They're typically low-budget productions made simply to capitalize on a popular trend in the movie industry. If everyone loves sexy vampire movies, then throw together a rough film for a few million and make some of that sweet blood money. Exploitation films are usually characterized by over-the-top sex, violence, or freakishness.
Exploitation films have lots of subgenres. Pulp Fiction draws briefly from the biker film subgenre, as well as the Blaxploitation movies of the 70s, like Shaft and Cleopatra Jones, which had all the same characteristics of exploitation films but were made with an urban black audience in mind.
This is the slang name for westerns made in Italy (because pasta) that were popular in the 1960s. These films made Clint Eastwood a star, beginning with A Fistful of Dollars. Director Sergio Leone had huge success with that movie, and more spaghetti followed. Eastwood said:
"I think [the Leone films] changed the style, the approach to Westerns [in Hollywood]. […] They made the violence and the shooting aspect a little larger than life, and they had great music and new types of scores. […]. They just had a look and a style that was a little different at the time: I don't think any of them was a classic story—like The Searchers or something like that—they were more fragmented, episodic, following the central character through various little episodes." (Source)
Fragmented, episodic, great score, larger than life violence—you can see why Tarantino described Pulp Fiction as a "rock 'n' roll spaghetti western with the surf music standing in for Ennio Morricone" (source).