Examples:
Romance: happiness, the reader falls in love with your characters;
Mystery: kickstart your reader’s brain – he’s looking for clues, and a slice of life;
Erotica: you’re titillating the reader (erotica’s HOT on the Kindle, so if you can write it, good for you. I’m rubbish at it myself…
);
via www.fabfreelancewriting.com
Your stories are "shelved" at Amazon according to genre.