(/the strong form for pervert or perverted/) All pornographic anime falls into the hentai category. Hentai titles all have strong sexual content (which may or may not have a censorship mask depending on if an uncensored version was released) but they can have surprisingly good stories (Gosenzo San-E), be very funny (Spaceship Agga Ruter), be cute (Elf no Waka Okusama), or a combination. Of course, they can be sick as well and there are tons of those titles out there.
Acquired Taste: These are titles which may take a while to get used to. As such, you should watch several episodes before making up your mind. FLCL, Di Gi Charat, & Serial Experiments Lain are examples of such titles.
