This thread discusses the change/how it works a bit.
If I understand correctly, STEEM is the cryptocurrency, while STEEM POWER is STEEM that's been set aside for the purposes of the community. It gives you the ability to vote, and appreciates based on an interest rate. Powering up STEEM moves it to be used for the community, and powering down STEEM POWER moves it out of the community to just cryptocurrency form.
STEEM DOLLARS I'm a little more fuzzy on but when you get a pay out it will be 50% STEEM DOLLARS and 50% STEEM, both of which can be converted back into STEEM POWER if you choose (STEEM BUCKS->STEEM->STEEM POWER). IIRC I think I saw STEEM DOLLARS would somehow be pegged to the USD or something to that effect... that could be horribly wrong though.