Hentai vs. Manga
 

What is the difference? I have seen samples of both, and they look the same. Is this just two different names for the same thing?

Hentai is pics.
Manga is comics.

I always thought Manga was the gerne, and hentai was the porn part of it. *shrugs*

as far as I've been told it works like this:
Manga is just a clever word that lets you know that it is most likely a japanese comic - or at least looks like one. Traditionally read from right to left. You know, for fun.

Hentai aparently is derived from a word in japanese similiar to our word for pervert. So you can be a hentai - which you probably are. :)
"Hentai" just lets you know that the picture you're about to see is going to be a) a drawing of japanese style and b)nude in some fasion.

Manga is a general term for any Japanese comics.

Marburg is right with hentai = "pervert"... it's just a word people use to describe any form of pornography (picture, comic, animation, game, etc.) done in the style of Japanese cartoons.

Keywords to Japanese cartoon stuffs:
http://www.urbandictionary.com/define.php?term=anime
http://www.urbandictionary.com/define.php?term=manga
http://www.urbandictionary.com/define.php?term=ecchi
http://www.urbandictionary.com/define.php?term=hentai

Taking into consideration the cultural references and not actual hard-nosed definitions:

Manga - anime comics, both general audience and adult
Hentai - anime erotica, both animated and not

so you have have hentai manga or just plain ol' manga