Genre is what books, movies etc. can be categorized into. For example some genres include non-fiction, drama, fiction, etc. Fahrenheit 451 falls into the fiction category as it is not a real society and has many made up characters and technological devices. The subgenre of Fahrenheit 451 is Science fiction as it is set in a future where the technological advances shape the society. Fahrenheit 451 is also considered science fiction because the characters are made up, other than a few historical references such as Einstein and Abraham Lincoln who are not characters within the book. The themes deal with good and evil and is based around technology. The society is very fast running, where books and thoughts are limited and everything is entertainment based. The book also takes place in the future, where technology at the time of when the book was created was likely only dreamt of.
http://www.locomotiveonline.com/locomotive/ray-bradbury-on-farenheit-451-and-the-art-of-science-fiction/