Orphan Black is the best thing on TV right now. Season 1 just ended, and you should watch it immediately. But, do everything you can to avoid learning anything about it first! Here, just watch the first three minutes of the first episode. That should tell you everything you need to know to get sucked in:

One actress is playing multiple roles, and she does a really good job characterizing them as completely different people. Not quite as good as Toni Collette did in United States of Tara, but in that ballpark.

Once you've gotten to episode 3 or 4, it's probably safe to read about it:

Tatiana Maslany discusses all her roles:

Just technically speaking, how do you film one of these scenes, when there's three or four versions of you in the same space?

TM: It takes, like, 12 hours. [Laughs.] It's the most technically ridiculous time of my life. We have a camera that's called the technodolly, which basically memorizes a camera move internally, so that it does the same thing every time, which is awesome, so we don't always have to just do a lock-off shot, where it's just, like, two people standing in the same shot. The camera actually moves, and I think it really sells it. So the first pass we do to memorize the camera move, we use doubles in place of the other characters that I'm talking to. Then, once we have that, they leave, and I start to do it for real, just to eyelines, with an earwig [earpiece], which is saying the other lines, so the rhythm is the same. Then once we get a perfect take of that, I leave and come back as the next character and then shoot it from the other side. Now, I'm having to remember where the eyelines were, where I was before, when I stood up, how high I stood up, or how close I came, or if I handed myself a bottle of pills or something. It's heavily technical and probably the most challenging thing I've ever done on screen.