Manga are often called Japanese comic books, but this is usually not the case, since manga is an entirely new media unto itself.
Manga is a Chinese adapted word meaning "In spite of oneself picture".
Manga is best described as:
1. Japanese Black and white graphical pictorial novels with many, many pages.
2. Japanese still animation in picture form. (this tells the feel of manga the best.)
Manga is also singular and plural in it's form. e.g. One manga, ten thousand manga. There is no such word in Japanese as "Mangas"
Manga consists of:
- Black and white pictures
- Many pages per installment (usually 30+)
- Movie-like effects.
- Developed characters and plots.
Not all manga are marketed to a younger age group, as not all American comics are about Superheroes in tights.
Usually manga are serialized in either monthly, bimonthly, or weekly magazines. If a particular manga gets popular it will be serialized into tankoubon form, which is a smaller form of the original-sized manga.
If the tankoubon and the series keeps getting popular it will eventually be serialized into an anime. It is the mangaka's choice to have their manga made into an anime and usually gets say in the production of the anime.
The range of involvement with the anime for the mangaka can vary from no involvement, just sending in the basic plot, writing the scripts for the anime, choosing the director, producing the anime, to forming and having their own company make the manga. It all depends on the popularity of the mangaka in question and the company that is producing the manga.
Manga ranges in the plots and genres that it carries. There are generally 4 major genres:
Shounen- Boy's manga
Shoujo- Girl's manga
Otoko- Men's manga
Josei- Women's manga
There are also other loose categories, such as those manga for people in high school and the manga that is neither female or male oriented. These genres also tend to apply for anime also (but that's usually because of the manga it's based on). The major genres of the ones I listed are Shoujo and Shounen. Men's and women's manga in Japan is a new innovation, with the women's manga coming first chronologically.
Shoujo became popular with the publication of The Rose of Versailles by Ikeda Rikoyo who made the characteristic huge big eyes and soft liquid feel. This manga was so popular that in some schools it was used to teach about the French Revolution. Ikeda-sensei used romance plots which mark the majority of Shoujo today. Though Shoujo was originally known for the big eyes, the eyes have been getting smaller, and smaller, though they still hold the liquid feel and plotting techniques of the original shoujo. (The lack of big eyes is usually made up with more realistic features and more details oriented that way)
Some sub-genres are: Science Fiction, Fantasy, Love stories, Magical girl, Fighting manga, Comic Science Fiction, Interest manga (Such as volleyball manga, skating manga, sushi-making manga, or racing manga), and many other genres that could probably fill the next few pages.
Some genres and things in manga can be objectionable to parents, but as with any media it can range from cutesy to sadistic and bloody. For example there are two sub-genre of Shoujo manga that are called Yuri (Shoujo-ai) and Yaoi (shounen-ai) which deal with homosexuality.
Generally manga meant for children, such as shoujo and shounen manga don't show anything explicit with sex, or a lot of gore and violence. This is not to say that the manga meant for men and women all contain these things.
The nudity, sex and violence are usually used to further the plot. Like in America, they may promise these things just to attract audiences, though it may not have as much as promised. Not all manga have these things and not all manga have the same levels, though some have it even though the manga is aimed toward younger children.
manga that is meant for men and women often contain themes that children would not understand. This is not to say there is not hentai or perverted manga out there, but it is aimed at an older age group. Not all manga meant for men and women are hentai.