STEEM - the main currency, you can buy, sell, trade on the exchange, send to another user.
STEEM POWER - currency for investors, people interested in Steemit development.
STEEM DOLLARS - equivalent of US dollar exchange rate is not fixed, it does not reflect the real value. Now 1 STEEM DOLLARS = 0,83 $ US. This is misleading beginners users Steemit, may disappoint them.