So as you said there is Steem, Steem Power and SBD (Steem Dollars)

SBD is short for Steem Dollars (or Steem Backed Dollars to be precise). Their purpose is to be worth at least 1 USD in Steem at any time (right now they are worth more than that). They can freely be traded with others. You can buy SBD from the built in market with Steem.

Steem can also be traded freely, but it can also be used to Power Up your account. You can power up any amount of Steem to the same amount of Steem Power. You can also buy Steem with SBD from the built in market.

Steem Power increases your voting power. Basically the more Steem Power you have the more impact you have on the value of posts when you vote on them. Steem Power cannot be traded freely, but you can Power Down your Steem Power back to Steem if you want to trade it, however doing so takes up to 13 weeks if you want to Power Down all the Steem Power you have on your account.