I was in your shoes just 5 days ago. It can all seem very complicated at first. I suggest you read through the FAQ because most of your questions can be answered there. But here's the bullet point version:

1. Steem Back Dollars (SBD) - Are designed to be roughly equal to 1 USD (can vary depending on market conditions) and add a level of stability to the Steem economy.
2. Yes
3. Each of your keys have different levels of permissions associated with them. When performing low level interactions use a low level key. If the key is ever jeopardized no serious harm can be done with the access key.
4. I don't understand the question.