Hentai is the english name for a form of manga or anime that explores explicit sexual themes.
Hentai in japanese means 'perverted'.
the japanese term for this kind of material is "H" which they pronounce 'Ecchi'.
'Ecchi' has been taken into english to refer to Anime or Manga with slight sexual suggestion but no actual explicit material.
these naming conventions obviously cause a bit of confusion among some people.