What's the difference between Steem, Steem Power & Steem Dollars?

I recently read a post comparing Steem to a power plant. While I loved the effort to incorporate a topically relevant analogy, it actually did little to help me understand the difference between them.

Mainly, I'd love to better understand the difference between the two currencies currently being offered.

What's the difference between the STEEM and STEEM DOLLARS currently available?

Which would be a better investment and why?

Any insights would be helpful.

Thanks in advance.