Sure, so STEEM is the base currency of the network. It's a coin just like Bitcoin or Dogecoin. Economics on the Steem blockchain start and end with STEEM.
Steem Power (SP) is illiquid STEEM. It's STEEM you've locked up in a vesting balance which you can recover as STEEM 1 year after powering up, or up to 2 years if you take partial amounts early. SP is diluted at 1/10th the rate that STEEM is, to protect long-term stakeholders from the brunt of inflation, and your voting strength is proportional to your SP balance.
Finally, there's Steem Dollars (SD). An SD is a smart contract, which basically says that the Steem community as a whole owes you $1 worth of STEEM. The blockchain creates SD to pay content creators for quality content. SD is liquid, so you can transfer or sell it, and since it represents $1 of debt, it should generally trade for at least $1, maybe more depending on how bullish the market is on Steem. Alternatively, you can execute the smart contract, which causes the blockchain to lock up your SD for a week, and then print $1 worth of STEEM (at market rates at the time of printing), paid to the one who held the SD. This does dilute the STEEM supply, so to incentivize people to hold their SD rather than execute it, the blockchain pays interest on SD balances.