Steem Dollar (SBD)

The Steem Dollar (aka Steem Backed Dollar or SBD) is a token that is meant to be worth about $1 USD, making its value much more stable compared to STEEM (see STEEM Tokens). To be a bit more technical, a Steem Dollar is a promise to pay one dollars worth of Steem when it's redeemed.
The benefit of Steem Dollars is that business can accept them more comfortably since they have a more stable value. Can you imagine being a shop owner and trying to sell your $50 product for STEEM which a couple months ago would have been for 500 Steem compared to today being less than 50. Pricing their products and services are much easier when using a stable value, as well as knowing that what they've already been paid, won't loose value.
To try to keep the SD around the $1 mark, Steemit's Witnesses (see Steem_Witness) can change the interest rate paid (quarterly) for holding them in your wallet. Lately this interest rate has been 0% with the SBD price being higher than desired. When it gets near or under the value of $1 USD, we will start seeing the interest rate increase.
Steem Dollars may also be converted into STEEM in a process that takes 3.5 days.[1] But there is a quickier option using the Internal market.