On a five hour layover in Moscow, changing planes but not airport, would a U.S. citizen need a transit visa?
Note: The answer was valid at the time of posting in May 2014. SVO has expanded drastically in the last few years and what was not available 5 years ago may well be available now.
Original answer: As an American citizen that has travelled to Moscow and via Moscow many times, I can definitely confirm that you do NOT need a transit visa if your connecting flight is from the same terminal. If you have to change the terminal, then you do need a transit visa and you must get this visa before travelling - you cannot get it once in Moscow.
I am flying from LAX USA to Israel, via Moscow. On X'Mas. With Aeroflot. I have a 4.5 hr layover in Moscow. I am an US citizen. I asked the same questions the Aeroflot representative. He said, "if you dont leave or exit the airport, you do not need an entry visa. But if you want to go visit the city, during your layover, you do need a visa"