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Seinen: Seinen means ‘Young Man.’ Novels that specifically targets young adult males around the ages of 18 to 25 are Seinen titles. Typically the story lines deal with the issues of adulthood.
Ecchi: Possibly the line between hentai and non-hentai, ecchi usually refers to fanservice put in to attract a certain group of fans.
Mature: Contains subject matter which may be too extreme for a younger audience. Content that deals with mature themes such as gore, sex, or violence.
Adult: Contains mature content that is suitable only for adults. Titles in this category may include prolonged scenes of intense violence and/or graphic sexual content and nudity.
Smut: Erotic work of fiction that’s mainly targeted toward females. Smut stories often gives heavy focus on love and relationships between the main characters.
In general…
Sienen is not related to the presence or absence of sexual content.
Ecchi is the type of borderline "fanservice" that needs to die out.
Mature is for heavy stuff not directly showing sexual content (though it may be implied, such as a character having to deal with rape or similar).
Adult is for stories that show stuff, but don't make it the main focus.
Smut stories focus heavily on sexual content.