I’ve noticed that a lot of people confuse manga categories all the time. They can’t tell the difference between shoujo, shounen, seinen, and josei. I admit its rather difficult at times because there are crossover mangas and etc., but there are differences. Genre such as fighting, sci-fi, school drama etc. is not what determines which category a manga fits in. Although admitedly there are more school drama shoujo mangas than shounen and more “hero” type shounen than shoujo. There is a difference in drawing style. Also Seinen and Josei are not commonly published in the United States, because they target older audiences.
For starters shounen manga is target towards boys (although from what I hear more girls than boys read shounen jump). Manga like Dragonball Z, Naruto, and Claymore are shounen. Shounen is more restrained than shoujo in the flow of movement. The flow is easier to read and the characters are more likely to stay in the boxes. Also, the background is either drawn out or is full of sound effects and/or speedlines.
Shoujo manga is targeted toward girls and is most often about romance. The backgrounds are more flowery (not all the time of course) and the people are more beautiful. Examples of shoujo manga include Fruits Basket, Ayashi no Ceres, and Ouran High School Host Club.
Seinen is basically men’s manga. Examples include such various titles as Elfen Lied, xxxHolic, and Monster. This is the hardest category to determine. Clamp has drawn beautifully in xxxholic which is seinen. Seinen also includes manga with a lot of nudity and panty shots, but that is not in all seinen. In general seinen is more mature than Shounen.
Josei is essentially women’s manga. Emma, Paradise Kiss, and Nodame Cantabile. The romance is not as as “romantic” as shoujo and tends to be more realistic. Also Yoai mostly falls under this category.
Remember you are not restricted by you sex from reading manga from different categories. These are just who they are targeted to. In general Seinen and Josei are about more mature themes that usually wouldn’t interest younger people. Shoujo is more about things that girls think about and Shounen is more about what boys think about or at least are thought to think and dream about.