So SBD are the steem equivalent in dollars. They fluctuate with the steem/usd exchange rate. You can transfer those immediately into Steem via the marketplace, or transfer them over a 3-day period through Steemit. Steem is the real cryptocurrency accepted in the real world. Steem Power is really unrelated to Steem, though you can buy Steem Power with Steem. Lots of tutorials on here, and I am not sure I even have all of it.

However, the reason I chose to trade in SBD, is because it is how we earn with posting and curating, but (as I understand it) it only applies here within steemit.