How does SP work?

Forgive my newness, but I'm sure I'm not the only one. I am having a hard time figuring out how Steem is actually going to be worth anything. Haven't had time to read the white paper yet. Can someone answer a few questions?

1. What is SP vs Steem? I understand that SP is the muscle behind your votes- high SP makes your voting influence a higher payout, and having high SP makes you a whale. Steem is the asset you can covert into cash. What else do I need to know?

2. How do you convert Steem into dollars? Do I need to link my Steem account to a bank account? That sounds suspicious.

3. If I want more SP, how do I get it? I know I will have to buy it, but how, and how much does it cost. How much would it cost (in dollars) to have enough SP that one might consider me a whale?

Thanks for the time. Airborne!