Steem Power (SP) is just steem locked up in a way to affect your voting power / payouts. The more you have in SP, the more you get from upvotes and also the more your are able to give out VIA an upvote. It is your POWER on the site. If you put Steem into SP, it takes 13 weeks of equal payments to exit SP back into regular Steem that you can sell on an exchange. Most people put everything into SP if they are long on the platform (like me).
Continue to build up SP. Accumulate, accumulate, accumulate. Key to success.
Steem Dollars (SBD) are "Steem backed dollars" used to help assist with the economics of steem, it creates liquidity and also helps stabilize Steem value. People on here use SBD to pay for services like 
Steem is just steem. If you have STEEM, you have the actual blockchain asset this entire network is based upon. It's like owning a share in Steem's future.
If you want to learn more about STEEM. Please click the link below:
The official website: https://steem.io
Wikipedia: https://en.wikipedia.org/wiki/Steemit
Community App: https://steemit.com
Steemit App Center: http://steemtools.com
The official blog: https://steemit.com/@steemitblog
Block Explorer 1: https://steemd.com
Block Explorer 2: https://steemdb.com
Github: https://github.com/steemit
Chat: https://steemit.chat
Message Board 1: https://busy.org
Message Board 2: https://beta.chainbb.com
Announcement: https://bitcointalk.org/index.php?topic=1466593
Videos: https://www.youtube.com
Market: http://coinmarketcap.com/currencies/steem
Market: http://coinmarketcap.com/currencies/steem-dollars
You can get STEEM & STEEM DOLLAR from the following exchanges:
https://bittrex.com
https://blocktrades.us
https://www.livecoin.net
https://openledger.io
https://www.coinspot.com.au (Buy and Sell Steem using Aussie Dollars)
https://changelly.com
https://www.coinpayments.net
https://poloniex.com