Steem vs SP vs SMD?

Hi, I was wondering if someone could give me a technical description/explanation of STEEM vs SP vs SMD? Or verify that my understanding is correct.
Steemit is a blockchain-based social media platform.
STEEM is a blockchain-based cryptocurrency token that can be bought and sold on a digital currency exchange.
SP is the "currency" given, used, and traded on Steemit that can be "sold" for STEEM.
SMD can be purchased as an investment, like seed funds, into STEEM (or Steemit?) and is paid back in STEEM (or USD?) after the close of a round of financing?

Is there a finite amount of STEEM, SP, and SMD? Is STEEM any different than bitcoin? Is STEEM open for public mining?

When I joined Steemit, I was given some SP to "spend", where did this SP come from? If there is a finite amount of SP, who's pocket did it come out of? If SP is not necessarily finite, but STEEM is, and SP and STEEM can be exchanged, then does that mean for each new user to Steemit who is given SP, the value of STEEM inflates?

Thanks for the clarification!