Both currencies steem and sbd can be withdrawn anytime to an exchange and converted to Bitcoin and then back to usd $ to your bank account. Main one is steem , once you buy steem or earn steem you can power up and convert it to steem power which gives you more influence and voting power and other people will start paying more attention to you because they want your vote which is now worth more, so having steem power helps you attract more votes. So having 500 steem power makes your vote worth 10 cents instead of 1 cent. You can vote 10 times per day and your voting power regenerates that much per day. Withdrawing steem power takes 13 weeks incrementally goes back to regular steem you can withdraw. Steem dollars aren't used as much it's basically like just converting your steem to a tether, a place that locks into a usd equivelant to lock in StEEM to stop volitility without having to withdrawal it all the time