Hentai (hen = 'weird', tai = 'perspective/view') is the misinterpreted Japanese word for "pervert" or "perverted", and is used by the western world to describe sexually explicit anime, manga/doujinshi, video games (aka eroge), and other Japanese media of similar nature. Sometimes these can have actual plots, but much of the time they are as devoid of such a thing as most western pornography. Hentai material can focus on tender moments between high school students who are discovering passion together for the first time. It can be sweet, tender, and airbrushed, or it can be ghastly, gory, grotesque things that'd horrify a Castro Street leatherdaddy.
Material that is less explicit or intense is called "ecchi" ("H", pronounced "eh-chee").
It should be noted that both of these usages are specifically western. In Japanese, "H" is a catch-all term for all things naughty. It can be used as a noun, adjective or verb as the situation requires. When used to refer to porn, there is no implication of the degree of lewdness.
"Hentai", on the other hand, is not a word that is ever used to refer to porn in Japan, although in manga and anime it is used as an insult ("pervert"). Japanese people tend to find it extremely hilarious or just plain confusing that English speakers refer to Anime porn as "hentai"; of course, using the letter of a foreign language to refer to it is pretty baffling too. In general Japanese usage, hentai means "strange", "perverse", "transformation", "mutation" or "metamorphosis" and can refer to things that are entirely nonsexual. Note that there are two words pronounced "hentai"; the other means "strange shape" or "deformed"For example
It should be noted that "perverted" in this case can mean as to either manner of preferred sexual conduct or the desire therefore — a man who excessively desires sex with women (or a woman) can be told off as "hentai" as much as a man with moderate desire for men (post-Meiji). (Western mediæval and Renaissance morés were similar: sodomy was often seen as being the culmination or product of an undifferentiated and excessive weakness toward the sin of Lust; old reprobates who were too slow to hire women were thought to hire boys.)
Definitely not to be confused with Sentai, though Rule 34 has made sure that there's some overlap between the two.