Steem, Steem Dollars, Steem Power

Hi! I'm a newby and I'm trying to understand the mechanics of the three Steem tokens. I think I get the purpose behind Steem Power. As I understand, it as an asset or equity stake in Steemit utilized to maintain the Steemit community. Steem is an inflating currency used primarily as a means to rewarding curators and creators. It also serves as an exchange between other currencies. What I'm struggling with is the purpose behind Steem Dollars. I believe the whitepaper claims that Steem Dollars are used to provide the community with an instrument pegged to the stability of the US dollar. It states that Steem Dollars represents a debt? If so, who is the counterparty and why is it so important to maintain a connection to a fiat currency? Also, I recently noticed that my humble Steem Power balance seems to go up a little every day or so? How is this? Any links or replies explaining these concepts would be greatly appreciated. Thanks!