{romance} These stories are all about Him falling in love with Him (or Her for you het people). These stories are usually sweet and fluffy and often involve poetry, kissing, and romantic dates.
{smut} Not quite the same as romance, these stories are all about Him banging Him (or Her for you het people). Remember the lemon type? Lemons are always in the smut genre. Most smut is NC-17.
{humor} One of my personal favorites, any story in the humor genre is comedic. They set out to make you laugh. In J-Rock you're laughing at your favorite rocker's expense, but who cares? They're uplifting, shallow, and fucking fun.
{angst} Ah, exactly the opposite of humor, angst stories are sad, depressing and dramatic. They often deal with huge things like death, loss, illness, unrequited love, and abuse. Half of the angst stories out there have happy resolutions, but half of them don't. You never can tell. Angst stories are never G and are, in fact, usually R.
{horror} Horror stories are the kind that put you to sleep with nightmares. They usually involve mystery, suspense, monsters, the supernatural, phschology, murder, and frightening descriptions. They're good reads, but it may be wise to not read them late at night, when you're home alone, or right before going to bed. And if you do read them don't look in the mirror for at least 24 hours.
{mystery} A step down from horror, mystery stories often involve things like murder, theft, espionage, and other exiting elements. In mystery stories there may be some scary things, but for the most part they're just suspenseful and a hell of a lot of fun.
{action} While it's hard to write an action story (words just don't have the same effect as moving pictures), they are very fun when they come around. These are usually rated PG-13 and often involve a lot of blood, gore, and other gross things. Martial arts and guns are common in these stories and they are usually pretty quick-paced.
{fantasy} These can be anything from J-Rocker learns how to cast spells, to J-Rocker falls down a well and ends up in the middle of a battle between Inu-Yasha and Sesshoumaru. Usually, though, fantasy stories involve wizards, enchanted places, and fairytale creatures.
{sci-fi} Science fiction stories are similar to fantasy and can feature something like a scientist's creation falling into the hands of the innocent Takanori or Dir en grey being chosen to be the first band to play in space. These usually take place in outer space, have lots of creepy monsters, and include the characters using technical jargon that makes no sense to you or me.
{poetry} If a story is poetry it isn't really a story, is it? These are exactly what you think they are: poems.
{general} This basically means that no genre fit that story. It's just your average, all-around tale.
Fanfictions can and will often be placed in more than one genre.