Who is torturing Theon Greyjoy and why?
He comes out of no where and I haven't read the books. He just keeps torturing him and giving hints on who he is...
It's worth noting that this happens much the same way in the books; we don't fully know what's happening until somewhat later, so it should be considered as part of the plot and not something that you're supposed to know right now.
Aaargh. If you don't know who he is, maybe it's because you are _not supposed_ to know yet! How about you relax, watch the show, and find out in due time? Why the need for spoilers? :(
And now, by reading the answers below, you've spoiled yourself of a surprise. Well done!
Question is, why do they spend so much air time showing this elaborate torture? Enough already.
@TLP - I would say the answer is two fold. Theon has nothing to do until much much later, so it's a way to keep him on screen. And right now Theon is one of the most hated characters in the show, second only to Joffs. The reaction of his haters has been interestingly polarized.
Answer in spoiler:
Roose Bolton's bastard, Ramsay. Their sigil is the flayed man.
I don't know his name in english but in french it is "Schlingue", which could be translated by "Stinky" or "Smelly", does it help you remember his name ?
Spoiler alert :
Roose Bolton's bastard : Ramsay Snow. Why? - He's not a very nice man.
All bastards in the North are given the name of Snow. However be warned - he has been legitimised now and prefers Ramsay Bolton. If you address him as Snow you'll probably spend a year having your skin flayed off in the Dreadfort - or worse.
The other answers have given spoilers on his name. I'll give an explanation of his motivation without giving away too much (quoting myself here)
He's basically a psychopath. He loves torturing people. At one point he married a highborn lady for her inheritance of land. He then locked her up in a tower with no food. She starved to death -- but not before eating off a few of her own fingers.
He also captures girls, releases them naked into the woods, and hunts them. He rapes, kills, and flays them when caught.
He loves doing these things; it gives him great pleasure to watch others suffer (he's a sadist).
All this aside, there is a reason why he is doing this to Theon. He wants Theon to lose almost all sense of identity and remain as his meek servant. He also has some plans that will require Theon's help to be executed well.
Roose Bolton's bastard son Ramsay Snow. Remember, Roose told Robb that he could send his bastard son to recapture Winterfall? And Robb said OK, but he wanted Theon taken alive. So Ramsay captured Theon and took him to Bolton castle at Dreadfort. The Bolton's sigil is the 'Flayed Man' because they are proud to be known for torturing their enemies.
So there you have it.