Steem Dollars/SBD (Steem Backed Dollars) are meant to be pegged to USD. The cryptocurrency is STEEM, SBD is just a token backed by STEEM. SBD is meant to be stable, although it's currently not.

So every day STEEM (the crypto) is put in a reward pool, then everything in the reward pool goes out to the authors/curators and when it does it's converted to SBD, so the $ you see is SBD.

I think the problem is that most people use their SBD to power up their account, so the demand makes it go up in value :P