By far, the largest Bitcoin exchange where you can convert your currency into Bitcoin and back again is the one operated at mtgox.com. In a very distant second place is the Bitcoin exchange provided at BTC-e.com. Neither of those are based in the United States. The largest U.S. exchange is Coinbase.com.
The Bitcoin market is a very young ecosystem – barely 4 years old – that has yet to develop a mature, diversified infrastructure to support it. Mtgox.com used to be the only game in town when it comes to using a Bitcoin exchange, but now there are more than 70 Bitcoin exchanges around the world.
Depending on their country of residence, Members may not access all the functions of the Site.”
For the full terms of using the Bitcoin trading platform provided by mtgox.com, visit their page here.