Definition
Manga are comics or graphics novels originating from japan.
Traditionally, manga stories flow from top to bottom and from right to left. A mangaka is the author of a manga
In Japan, people of all ages read Manga. there is a lot of genres.
They are classified by demography,
there is the "kodomo" for young audience (less than 10 years old), the "shōnen" for a young male audience (from 8 to 18 years old),
the "shōjo", the same as the Shōnen but for girls, the "seinen" for an adult male (16 years and over),
the "josei" same as seinen but for women, and "seijin" for adult.
But this is not because a manga is in a demography that doesn't suit you that you shouldn't read it.
Manga are just claffified according to the age and the gender of the target readeship.
I'm going to explained the 3 most common categories.
Shonen
Shōnen manga is typically characterized by high-action,
often humorous plots featuring male protagonists.
Commonly-found themes in shōnen manga include martial arts, robots, science fiction, sports, horror or mythological creatures.
The camaraderie between boys or men on sports teams, fighting squads, and the like are often emphasized.
Protagonists of such manga often feature an ongoing desire to better themselves, and often face challenges to their abilities, skills and maturity.
None of these listed characteristics are a requirement, what defines whether or not a series is shōnen is the official classification of the magazine it is serialized in.
Shojo
Shōjo manga covers many subjects in a variety of narrative styles,
from historical drama to science fiction, often with a focus on romantic relationships or emotions.
We can find some genres such as magical girl, centered around young girls who possess magical abilities,
which they typically use through an ideal alter ego into which they can transform.
Or such as sport with female players, or such as yaoi which is is a genre of fictional media originating in Japan that features homoerotic relationships between male characters.
Like Shōnen, none of these listed characteristics are a requirement.
Seinen
Seinen manga can focus on action, politics, science fiction, fantasy, relationships, sports, or comedy.
have a wide variety of art styles and variation in subject matter.
If it takes up on the whole the themes approached in the shōnen, the plots are however more complex,
the characters more subtle, tortured or realistic.
Aimed at a more mature audience, this type of story is often more credible,
but sometimes also violent or tinged with eroticism, without being a generality because the subjects covered are very diverse.
like Shōnen and Shōjo, none of these listed characteristics are a requirement.