What genre is Robinson Crusoe?

Robinson Crusoe, written by Daniel Defoe, is a novel. A novel is a genre defined as a long imaginative work of literature written in prose. In other words, it is fictional rather than based on a true story and is written in prose rather than verse.
One can also talk about "subgenre" or what type of novel it is. First, it is a type of travel fiction, set in an exotic local. Next, it is a realistic story, rather than a fantasy or romance, in that it describes relatively plausible behavior by an ordinary person rather than heroic acts of someone with supernatural powers. 
In some ways it resembles a spiritual autobiography, as it shows the development of the religious and spiritual thought of its protagonist, but it is not autobiographical per se because it is fictional and autobiography is factual. 

Robinson Crusoe, published in 1719, is an early example of the genre of novel. It was composed to mimic the first-person account of a shipwreck adventure, with the words "written by himself" on the title page, so many early readers took it to be a factual account of a shipwrecked sailor's adventures alone (and later with a native companion he called Friday) on a deserted island. 
Although based on a true story, the work was the imaginative construct of author Daniel Defoe: there was no actual Robison Crusoe and while trying to be factually realistic in its detail, the story, if inspired by other reading, largely sprang from Defoe's mind. Because of its effort at realistically showing how a person would survive on an deserted island (no magic saves Crusoe) and the depiction of a psychologically realistic protagonist, the book is considered a novel rather than a romance.