@Literaticat has posted a list of genre definitions. Perhaps the most useful paragraph for beginners is this one (emphasis/shouting hers):
PLEASE NOTE: YA, MIDDLE GRADE, PICTURE BOOK, GRAPHIC NOVEL, FICTION, NON-FICTION & BIOGRAPHY ARE NOT GENRES. THEY ARE CATEGORIES. “Genre” is a further classification beyond category. If I were to use a Biology class analogy (bear with me, I had to go to summer school for Biology) I’d say that in the taxonomic hierarchy Kingdom-Phylum-Class-Species, “Kingdom” is book, “Phylum” is format of book (electronic, hardbound, paperback), “Class” is category (YA, fiction, etc), “Order” is big-genre, “Species” is sub-genre.
Then she wades right in with some examples:
URBAN FANTASY is always set in a city, and features um… FANTASY scenarios. For example, faeries that are addicted to drugs and live in the subway system. Or trolls who hang out in clubs and impregnate human chicks. Or whatever. If you haven’t written a dark and gritty fantasy set what we would recognize as a human-style city, you haven’t written an urban fantasy.
I agree with many of them, though not all. For example, I prefer the Historical Novel Society‘s definition of historical fiction:
To be deemed historical (in our sense), a novel must have been written at least fifty years after the events described, or have been written by someone who was not alive at the time of those events (who therefore approaches them only by research).
There’s also a lot left out. Where’s lesbian fiction? Where’s the Western? Novel of manners? The campus novel? And, oho-ho, The Great American Novel?
I think we could have some fun with this. We need both serious definitions (what exactly is ‘lesbian fiction’?) and more playful varieties. Here’s a Devil’s Dictionary-inspired description to get you started: Noir, the horror fiction of the crime genre.
What genres (and subgenres, and sub-subgenres) would you like to see defined?
—
Nicola Griffith 
Some thoughts (as I'm not quite awake to actually be coherent).
Is there a sub-Kingdom “well written” and “not well written?” Or “Fun” and “Not fun.” And what about “Beach/Airplane Reading?”
I'm wondering how she is saying that Mysteries are not Fiction. It seems to me that Fiction and Non-Fiction would be above Genres.
Instead of or in addition to “Historical Fiction” you could have “Period Fiction.”
From working in a bookstore, I can tell you that people ask for “African American Fiction, GLBT Fiction, and Religious Fiction.” There are also several types of Romance Fiction (Historical, Modern, Paranormal, etc.).
And speaking strictly as a Biology major, it's kingdom, phylum/division, class, order, family, genus, species (then sub-species, varieties, races, etc.). So, sticking with the taxonomy metaphor…
KINGDOM: Book, Graphic Novel, Film
SUB-KINGDOM: Trade Paperback, e-book, etc.
PHYLUM: Fiction or Nonfiction.
SUB-PHYLUM: Mystery, Science Fiction, Fantasy
CLASS: Mystery, Crime Fiction, Cozy, Thriller
ORDER: Noir, Locked Room Mystery, Police Procedural
FAMILY: Period Noir, Contemporary Noir.
GENUS: Spade & Archer
SPECIES: Joe Gores
As to “lesbian fiction,” that can be either a category or a pejorative. Nicola, do you consider your novels lesbian fiction?
DianneorDi, Yep, she edited the whole kingdom–>species thing. But that's what people in publishing do :)
Not sure I agree with your example, exactly. For example, I'd say 'Crime Fiction' is further up the hierarchy, with 'Mystery' as a subset. (I'd also put something like 'arealism' on the same level as 'crime fiction', with subsets such as 'fantasy', 'sf', 'alternate history'.) Maybe. Hmmm…
As for 'fun' or 'not fun' maybe that's outside the classification system, just as 'edible/inedible' and 'hunter/hunted' are.
ElaineB, sure. But that's not all they are. They're also–variously–science fiction and crime fiction and historical fiction. They're literary fiction and thrillers. This is why I'm a big fan of metadata–that way, I use all the labels that apply, not just some.
The whole taxonomy metaphor is flawed, since you can have Young Adult Mysteries and Adult Mysteries (and GLBT Mysteries and African American Mysteries). You cannot have, for example, a mammal that is plant or a possum that is a bird.
I'm with you about metadata and the fluidity of genres. I've never intentionally read “GLBT Fiction.” I read TBP because the story intrigued me. The fact that the protagonist (or author) was lesbian really didn't enter into the decision.
Well, I look for lesbian fiction–about lesbians if not by them (though by is preferred)–after a lifetime of reading about heterosexuals, I'm tired of them. But within that, I'm interested in “beach reads” as well as “literary.” I like mysteries, science fiction, contemporary, you name it–as long as it has believable lesbian characters.
I felt a stab of wrongness when I saw that this classification put hardcover / paperback / e-book so near the top/base. I don't think those are important at all in classifying books, especially since the same content can appear in all three formats. To keep that distinction I think one would also have to add a higher/deeper classification depending on whether the book is handwritten or printed, and then perhaps whether on paper, parchment, vellum, papyrus, and then codex or scroll … and what about clay tablets? These can be important for cataloguing a library or other collection, but not for literary discussion. (You know that in the old days books were classified by size, from largest to smallest? Of course you do.) I'd just as soon leave format out of it altogether, except in cases where it's directly pertinent.
And I agree with DianneorDi that this kind of classification system doesn't work with the content of books anyway, for the reason she gives. Organisms that straddle classifications are a problem for the Kingdom/Phylum/etc. kind of system, but books that straddle classifications are manifold and possibly the norm.
Promiscuous, yep, metadata 'r' us.