Actually, it has surprisingly little to do with Quentin Tarantino.
Initially more a format than a genre, Pulp Fiction was a general term used to describe the stories published in pulp magazines – cheap magazines printed on rough, wood pulp paper (magazines printed on smooth, high-quality paper were called “glossies”). It was not a complimentary description.
Pulp fiction was synonymous with run-of-the-mill, low-quality literature that got by more on its cheap thrills and lurid details than on any merit.
It was, of course, wildly popular.
At least until WWII, when paper shortages and rising production costs spelled the end for many pulp publications.
Today pulp fiction lives on mostly as an homage to those early 20th century writers and short stories. Pulp fiction, whether it be sci-fi, adventure, crime fiction, etc., had a certain sensationalist feel and many common themes and elements that developed over the decades of their popularity and some modern authors (especially hard-boiled crime authors) have taken those elements and brought them to their books and stories today. Or movies (there’s your Tarantino-tie-in).