what is SBD steemit currency? how to reduce delegated steem power

What is SBD?
sorry for the noob questions

1. I really dont understand the difference between the SBD and a steem dollar/steem.
   I was given a tip which shows in the wallet history as a SBD. Also where does this appear in the wallet balances?

2. Can I reduce the steem power that is delegated?

Thank you.