Steem
Steem is the crypto currency that is meant for being used in transactions, therefore inherently a (virtual) currency. In the future Steem will also be used as a baseline to 'create' new smart tokens. As the valuation of the Steem block-chain fluctuates the value of a single tokens can also fluctuate. Steem can be traded freely between different steem addresses.
Steem Dollars
Steem dollar is a separate coin as opposed to Steem. And is meant to have a stable, fixed, value. Of around 1$. In times when the valuation of steem is too low, or too high, and therefore doesn't accommodate day-to-day transactions. SBD can then be used instead. Like Steem, SBD can be transferred between different accounts.
Steem Power (SP)
By the processing of powering up a user can gain steem power. The acquisition of steem power can be seen as taking a stake in the Steem block-chain. Steem Power is tangled to a single 'account' and cannot be traded. By the process of powering down, Steem power can be interchanged for Steem. This however is a very costly process that takes over two years.
To define something as a currency. it must be used in circulation as a 'medium' of exchange. As Steem Power cannot be circulated. Steem power, should not be seen as a currency on it's own.
