@john,
Shonen - The term shōnen, shonen, or shounen manga (少年漫画 shōnen manga?) refers to manga marketed to a male audience from the age of 10 and older. The Kanji characters (少年) literally mean "few" and "year", respectively, where the characters (漫画) generally mean "comic". The complete phrase literally means "young person's comic." Examples include Dragon Ball, Naruto, Bleach, Hunter X Hunter, One Piece, Saint Seiya, Fairy Tail, YuYu Hakusho, Detective Conan, Yu-Gi-Oh and Fullmetal Alchemist.
Shōnen (少年) manga (漫画) is typically characterized by high-action,[1] often humorous plots featuring male protagonists. The camaraderie between boys or men on sports teams, fighting squads and the like is often emphasized. Attractive female characters with exaggerated features are also common (see fan service), but are not a requirement; Dragon Ball, for example, has only a few such characters. After the case of Tsutomu Miyazaki, depictions of violence and sexual matters became more highly regulated in manga in general, but especially in shonen manga.
Shoujo - The term shōjo, shojo, or shoujo manga (少女漫画 shōjo manga?) refers to manga marketed to a female audience roughly between the ages of 10 and 18. The name romanizes the Japanese 少女 (shōjo), literally: "little girl". Shōjo manga covers many subjects in a variety of narrative and graphic styles, from historical drama to science fiction - often with a strong focus on human and romantic relationships and emotions.[1] Strictly speaking, shōjo manga does not comprise a style or a genre per se, but rather indicates a target demographic.[2][3] Examples include Ouran High School Host Club, Fruits Basket, Fushigi Yuugi, Sailor Moon, Vampire Knight, Romeo x Juliet, and Tokyo Mew Mew.
Seinin - Seinen manga (青年漫画?) is a subset of manga that is generally targeted at an 18–30 year old male audience, but the audience can be older with some comics aimed at businessmen well into their 40s. In Japanese, the word Seinen means "young man" or "young men" and is not suggestive of sexual matters. The female equivalent to seinen manga is josei manga. It has a wide variety of art styles and more variation in subject matter, ranging from the avant-garde to the pornographic. Seinen manga is distinguished from shounen, or boy's manga by having a stronger emphasis on realism and also by having a more well developed storyline. Because of the emphasis on storyline and character development instead of action some seinen series are often confused with shōjo, or girl's manga.[citation needed] This is especially true of seinen comedy series such as Chobits, and Chi's sweet home, or seinen drama such as Twin Spica. Other examples of seinen manga include: Gantz, 20th Century Boys, Blame!, Ghost in the Shell, Akira, Berserk and Battle Angel Alita.
A common way to tell if a manga is seinen is by looking at whether or not furigana is used over the original kanji text: if there are furigana on all kanji, the title is generally aimed at a younger audience. The title of the magazine it was published in is also an important indicator. Usually Japanese manga magazines with the word "young" in the title (Young Jump for instance) are seinen. Other popular seinen manga magazines include Ultra Jump, Afternoon, and Big Comic.
Source: Wikipedia