Steem Dollars (SBD) are not linked to your voting power. They are tied to the US dollar and so pretty much 1 SBD (Steem Dollar) = 1 US Dollar. There was supposed to be interest earned on SBD each year but the witnesses who control the interest rate have it set at 0% right now to keep the price down to $1USD.

Steem is the liquid part of the cryptocurrency that can fluctuate a little more in the price. It is worth about $1.40 USD today. It has been as high as $4 and as low as $0.06. This money can be sent to other users, to exchanges to cash in for Bitcoin, or exchanged for other services just like money.

Steem Power (SP) is where you get your voting power from. This essentially comes from Steem but it is locked in for 3 months. If you want to buy more voting power, you either need to buy steem from an exchange and send it to your wallet on steemit and then "power up" to SP. You can also buy it directly from your wallet.

If you are looking to get more voting power from your posts, you need to select "power up 100%" from the little menu on the bottom right side before you hit post. This will give you your entire payout in SP. If you don't check this, you will get some SP and some SBD.

The wallet section describes it all and there is a FAQ Section