Exchanges offer a way to trade Bitcoins (and altcoins) on a market (like the NY stock exchange).
BTC-E is a straightforward exchange with low fees:
BTC-e charges a 0.2% fee on each transaction carried out by users on the website. This transaction may vary and may be different for each individual account. USD/RUR – fee 0.5%
You can trade multiple types of Crypto-currencies (Btc, Ltc) in multiple currencies (USD).