Hentai means 'erotic' art. Wikipedia has a better explaination.
Hentai [SPAN style="FONT-WEIGHT: normal"]([SPAN class=t_nihongo_kanji lang=ja xml:lang="ja"]変態 or へんたい[/SPAN][SPAN class=t_nihongo_norom style="DISPLAY: none"][SPAN class=t_nihongo_comma style="DISPLAY: none"],[/SPAN] [SPAN class=t_nihongo_romaji]Hentai[/SPAN][/SPAN][SPAN class=t_nihongo_help][A title=Help:Japanese href="vny!://en.wikipedia.org/wiki/Help:Japanese"][SPAN class=t_nihongo_icon style="PADDING-RIGHT: 0.1em; PADDING-LEFT: 0.1em; PADDING-BOTTOM: 0px; FONT: bold 80% sans-serif; COLOR: #00e; PADDING-TOP: 0px; TEXT-DECORATION: none"]?[/SPAN][/A][/SPAN])[/SPAN] is a [A title="Japanese language" href="vny!://en.wikipedia.org/wiki/Japanese_language"]Japanese[/A] [A title=Word href="vny!://en.wikipedia.org/wiki/Word"]word[/A] that means "strange appearance" or "abnormality". However, in [A title=Colloquial href="vny!://en.wikipedia.org/wiki/Colloquial"]colloquial[/A] situations it often means "perverted" and is subsequently used in many other countries to refer to [A title=Anime href="vny!://en.wikipedia.org/wiki/Anime"]anime[/A], [A title=Manga href="vny!://en.wikipedia.org/wiki/Manga"]manga[/A], and [A title="Computer and video games" href="vny!://en.wikipedia.org/wiki/Computer_and_video_games"]computer games[/A] with explicit sexual or [A title=Pornography href="vny!://en.wikipedia.org/wiki/Pornography"]pornographic[/A] content (see [A title="Japanese pornography" href="vny!://en.wikipedia.org/wiki/Japanese_pornography"]Japanese pornography[/A]). The word is never used this way in [A title="Japanese language" href="vny!://en.wikipedia.org/wiki/Japanese_language"]Japanese[/A]; commonly used terms include "jū hachi kin" (18禁; prohibited for sale to persons under 18), "[A title=Ecchi href="vny!://en.wikipedia.org/wiki/Ecchi"]ecchi[/A]/[A title="List of H anime" href="vny!://en.wikipedia.org/wiki/List_of_H_anime"]H anime[/A]" (sexual/pornographic [A title=Anime href="vny!://en.wikipedia.org/wiki/Anime"]anime[/A]) "eroanime" (エロアニメ; derived from erotic [A title=Anime href="vny!://en.wikipedia.org/wiki/Anime"]anime[/A]), or "seinen" (成年; adult, not to be confused with [A title=Seinen href="vny!://en.wikipedia.org/wiki/Seinen"]青年[/A] young adult).
The term "hentai" is commonly used (outside of [A title=Japan href="vny!://en.wikipedia.org/wiki/Japan"]Japan[/A]) to refer to pornographic animation in general that is not necessarily anime or manga. This is most often the case if the said animation is an imitation of a pre-existing cartoon or character (e.g. [A title="Princess Jasmine" href="vny!://en.wikipedia.org/wiki/Princess_Jasmine"]Princess Jasmine[/A] hentai).
More: [A href="vny!://en.wikipedia.org/wiki/Hentai"]vny!://en.wikipedia.org/wiki/Hentai[/A]