It' really any text that is not fiction (an invented story or poetry). There are many different types of factual texts. Nonfiction is a kind or genre of writing based on facts and real life, not a fictional or fictional story. Nonfiction includes all types of information texts. A number of forms are available for non-fiction, including:
The GCSE bite size:
Nonfiction is a kind or genre: One class or kind of something.
Nonfiction covers all types of information texts.
Mystery - usually the investigating of a felony, usually one or more killings.