Yes, you buy and sell off-site and can transfer to/from your Steem wallet. If you just want to convert from SD to Steem, you can use the internal exchange.

I quote in dollars to make it simple to understand. Not many people know how much 0.000374 BTC is or what a "satoshi" is. And I also use dollars because that's how the wallet and internal exchange is priced.

There are three main exchanges where you can buy/sell your Steem: Blocktrades, Poloniex, and Bittrex.