There are 3 'tokens' on the Steem blockchain: STEEM, SBD (Steem Dollars), and Steem Power (SP)
STEEM
STEEM is the basic cryptocurrency of the Steem blockchain. You can earn it by posting, commenting or upvoting on Steemit.com. You can also deposit, withdraw or trade STEEM at any time.
STEEM POWER (SP)
STEEM can be powered up (or ‘staked’) as Steem Power (SP). The more Steem Power you have, the greater your influence on the platform. Having more Steem Power gives you more voting power, which allows you to earn more ‘curation rewards. Having Steem Power also gives you the power to vote for the witnesses who oversee the blockchain.
Steem Power can be converted back to STEEM through the process of ‘powering down’. This takes four weeks and you get a quarter of your Steem Power as liquid STEEM each week.
SBD (Steem Dollars)
SBD was originally designed to be on parity with the US dollar - so 1 SBD should equal 1 US dollar. However its value does fluctuate with the market although not as much as the value of STEEM, and it generally stays within about 10% of the dollar value.