Its not actually a genre but a target audience
Shounen = for boys:DBZ Naruto One Piss HXH
Shoujo = for girls:Ouran Host Club, Most Magical Girl anime, lots of romance.
Seinen = for adult man: Akira, Berserk, Hellsing, moeshit and harem.
Josei = for adult women. Lots of drama, and cute boys.