While I was originally researching STEEM, I couldn't find any answers to the difference between STEEM and STEEM Dollars. Well, it's pretty easily available under the FAQ section on Steemit.com.  Below is the breakout that Steemit provides to everyone in their FAQ section.

"What is the difference between STEEM, STEEM Power, and Steem Dollars?

STEEM - STEEM is the base liquid currency token in the platform. STEEM can be powered up into STEEM Power, traded for Steem Dollars, and transferred to other accounts. It is a cryptocurrency token, similar to bitcoin.

STEEM Power - STEEM Power (abbreviated SP) is a measurement of how much influence a user has in the Steem network. The more STEEM Power a user holds, the more they can influence the value of posts and comments. STEEM Power is less liquid. If a user wishes to “Power Down” SP, they will receive equal distributions of the STEEM weekly, over a 13 week period.

Steem Dollars - Steem Dollars (commonly abbreviated SBD) are liquid stable-value currency tokens designed to be pegged to $1 USD. Steem Dollars can be traded with STEEM, and transferred to other accounts for commerce or exchange. Steem Dollars may also be converted into STEEM in a process that takes 3.5 days. Steem Dollars can be used to buy things in marketplaces, such as PeerHub.com." - Steemit.com/FAQ