Having read "The Graveyard Book" by Neil Gaiman recently did make me wonder who or what was Silas' beside the obvious - Nobody's guardian.
According to Neil Gaiman's website Q&A:
Q: What is Silas in THE GRAVEYARD BOOK?
A: Silas is a Very Important Character in THE GRAVEYARD BOOK. Also, he is Bod’s Guardian.
That would seem to be the official answer.
Some sources on the internet, such as the LA Times, seem to think he is a vampire:
He [Neil Gaiman] also wanted to emphasize that, despite some trying times, he will not arrive at Royce Hall as glum as, say, Silas, the dour vampire who becomes an orphan’s protector in “The Graveyard Book.”
This discussion board post lays out some convincing evidence:
Silas is a vampire. He doesn't go out during the day, sleeps in a coffin, eats only one kind of food (and it's not bananas! Wink), avoids rain (in old myths vamps can't stand running water), and has no reflection. He's also directly stated as being neither living nor dead.
100% vampire. Although, there are lots of stories as to how he became a vampire. Some people say he is actually Dracula, and others say he was just turned into a vampire.
The Dracula theory is false, seeing how Dracula died in the 1800's. As for him being turned, I don't think that is true, either. He specifically stated that he had done horrible things to people in the past, and if I were turned, I wouldn't hurt people. I'd probably just hide in a corner and cry myself to sleep. Which leads me to this conclusion:
Silas is the son of Dracula. This makes sense. Dracula was killed by humans, and Silas, who was a loyal son, wanted revenge. After slaughtering people, something made him realize what he was doing was horrible, (Perhaps he met Miss Lupescu, who convinced him that what he was doing was wrong. considering he has a strange but settle fondness towards her in the book.) Silas wanted to make up for what he had done, so he joined the Honour Guard. He tried to return home, but couldn't, because people weren't too fond of him at the time. That is why he was in the old chapel, and that's how he met Bod. You don't have to believe me, but just consider it.
Wow. I had a totally different take. My feeling was that Silas was Death itself. You could take that line about him only eating one kind of food and postulate that the one thing he consumes is actually life. He is described as being something not quite from either world. His status among the ghosts and his power and influence over both the living and the dead seems to indicate he is something much more than a mere bloodsucker. His lines all reminded me of the narrator from the Book Thief.
Silas is definitely a vampire. He only eats one type of food (blood), says he has done terrible things in his past (probably turning other people into vampires), never goes out into the Sun, and has no reflection (in the caves under Krakow and when he and Bod went out for pizza after the Jacks were all gone).
Silas is not a vampire, because it also says he is neither living nor dead; so that means he's undead.
And yes, he does say he eat one type of food but he never said it was blood, and blood isn't a food, it's a drink because it's liquid.
It's also never stated he had vampire teeth because Bod and that would have realised (because who wouldn't) and he went on mini holidays all the time (don't think vampires go on that many trips without consuming at least one innocent life).