Take a look at my post here to get a foothold on the subject.

After that, ask some specific questions.

As for the difference between Steem, Steem Power, and SBD, it can be explained like this:

Steem = A token with a varying market value that can be used to purchase other currencies or to "Power Up"

Steem Power = Tokens in a sort of investment that give you influence in the Steemit community

SBD = Steem Based Dollar = A token intended to have a value as near the US Dollar as possible. (an aside the SBD has a tradeable value floating between $0.94 USD and $2 USD, because of this some think that we should just get Steem and abolish the SBD altogether)