Basic meaning on Steem, Steem Power, and Steem Dollars
Steem - It's the only trade able unit. You can buy/sell them at https://www.bittrex.com and convert it to Steem Power.
Steem Power - The power you have the more voting and earning power you have.
Steem Dollars - Each unit (SD) equal to one USD.
Please upvote and leave feedback.
Thanks.