...and, having answered your question "what is a literary magazine," poet and editor E. Kristin Anderson goes on to ask the question ‘What Is a Chapbook?‘
What is a chapbook, anyway? From looking at chapbooks, you'd think it's just a shorthand for a "cheap book"-- and they are, usually; at least, usually a lot lower in cost (and production values) than more highly polished perfect-bound books (although some chapbooks can be quite impressive handmade works of art). That's not what the word actually means, though: chapbooks were originally the books sold by a chapman, the itinerant pedlers that used to take carts from town to town in the middle ages, and would sell-- after the printing press was invented-- broadsides and cheaply-printed books alongside pots and pans and razors and nostrums. So, poetry chapbooks have a long history.
Today poets are their own chapmen, sell their own chapbooks, the itinerant peddlers of the middle ages, going from reading to reading, town to town, with their wagons filled with goods.