Manga is a style of Japanese comic books and graphic novels, typically aimed at adults as well as children.
Manga are comics created in Japan or by creators in the Japanese language, it conforms to a style developed in Japan in the late 19th century. Manga has always had a long and complex pre-history in Japanese art.
In Japan the term ‘manga’ is a term used to refer to both comics and cartooning. Outside of Japan, manga refers to comics originally published in Japan. In Japan, people of all ages read manga. The medium includes works in a broad range of genres: action-adventure, business and commerce, comedy, detective, historical drama, horror, mystery, romance, science fiction and fantasy, sexuality, sports and games, and suspense, among others. Many manga are translated into other languages. Since the 1950s, manga has steadily become a major part of the Japanese publishing industry. By 1995, the manga market in Japan was valued at ¥586.4 billion ($6–7 billion).(Wikipedia, 2008).

THE DIFFERENT TYPES OF MANGA
There are five types of Manga: Shonen, Shojo, Seinen, Josei, and Kodomomuke. These different types of manga focus on specific target demographic, they are not genres (action, romance, coming of age, etc.). Rather, they are just labels for who the intended audience is. So while the anime and manga that fall into each of the types can technically be of any genre, knowing which anime and manga you like fall into which type serves as a good road map for finding other similar titles you might enjoy.
(Eisenbeis, 2014)
All the five types will be expatiated on further:
SHONEN

Literally meaning “few years,” “shonen” (少年) typically refers to young boys under the age of fifteen. Thus, shonen anime and manga are aimed at that demographic. A lot of these anime and manga have a young male hero and are focused on action, adventure, and fighting. However, comedies and series with female protagonists are becoming more and more common. Shonen works often have more than a fair share of fan service as well.
(Eisenbeis, 2014)
SHOJO

“Shojo” (少女), which is often translated as “young girl,” is the female counterpart to shonen, and anime and manga of this type are aimed at girls between the ages of ten and eighteen. These tend to focus on romance and interpersonal relationships—though this does not mean they are necessarily without action or adventure.
(Eisenbeis, 2014)
SEINEN

“Seinen” (青年), on the other hand, refers to young men between the ages of 15-24. Seinen anime and manga tend to be of a more violent and/or psychological nature than shonen series—though, of course, there are seinen comedies as well. They can also have content of a pornographic nature (though this is not the focus of the work). Famous seinen anime and manga include such titles such as Berserk, Ghost in the Shell, Akira, Hellsing, and Gantz.
(Eisenbeis, 2014)
JOSEI

Anime and manga of the “josei” (女性) variety are aimed at adult women. Josei series are often slice-of-life or romantic tales featuring adult women, though, in recent years, shonen-like action-adventures have become popular as well. In general, these works tend to contain more realistic interpersonal relationships (as opposed to shojo’s often idealized ones) and can cover darker subjects like rape and infidelity. While many josei anime and manga have female main characters, many others have male characters. Male leads are usually of the pretty-boy variety and stories featuring them often have homosexual undertones.
(Eisenbeis, 2014)
KODOMOMUKE

Literally meaning “directed at children,” “Kodomomuke” (子供向け) anime and manga are exactly that. Kodomomuke works tend to be simple, imaginative stories that teach morals and other core values to young readers. Sometimes they are original stories while other times they are adaptations of Western classical literature. Popular children’s series include Doraemon, Hello Kitty, Chibi Maruko-chan, Heidi, Girl of the Alps, Anne of Green Gables, and Anpanman.
(Eisenbeis, 2014)
References:
Eisenbeis, R. (2014, July 3). How to Identify the Basic Types of Anime and Manga. Retrieved from Kotaku: https://kotaku.com/how-to-identify-the-basic-types-of-anime-and-manga-1538285518
Wikipedia. (2008, January 8). Wikimedia foundation Inc. Retrieved from Wikipedia: https://en.wikipedia.org/wiki/Manga