Tokyo Ghoul is about beings known as ghouls. They look human but have a unique but disturbing biology. They are creatures that are only able to eat human flesh to live. Their bodies won't accept anything and can actually kill them if they try.
It's not some undead/supernatural junk either. These beings are very much alive, very much kill able and surprisingly given a great deal of humanity to their characters.
The story focus' on Ken Kaneki who is attacked by a ghoul one knight but survives thanks to random beams falling on his attacker. He was fatally wounded though and the doctor that saved him used the organs of the ghoul to replace his. This, naturally, affects him horribly because it makes him half-ghoul. What the ghoul characters call a "one eye".
It is surprisingly very good. Check the thread out, I left a better impression of what I thought there. I read seventeen chapters and got hooked instantly.