1 SD = $1 USD (approx)
On our Steemit Wallet it states that tokens are "worth about $1 of STEEM."
So why would it be traded.. The value is literally always supposed to be worth $1.
Obviously currencies rise and fall in value, but the idea of the STEEM Dollar is to basically be Steemit's digital form of a dollar that is stable.
STEEM Dollars vs STEEM
On the reverse side of that you have just STEEM, which if you hold it in your wallet on your Steemit account I have heard that you are penalized.
So if you want to hold STEEM or trade it you should transfer to a website like Poloniex, which 
https://steemit.com/steemit/@ash/steemit-how-to-sell-steem-dollars-via-poloniex-newbie-friendly
The power of STEEM Power!
However if you are wanting to invest in STEEM the best way is to just Power up!
I have documented my SP interest in past posts which I have found that I earn almost 5 SP/ day, and the more you have the more you earn in interest.
(I have also been told that STEEM Power interest goes down to 10% after the first year.. so keep that in mind too.)
And as always thanks for taking the time to check out my post, and feel free to leave your own links in the comments!