See the FAQ. Did you find the menu in the top right?

Steem is the basic currency of Steemit. A fixed amount is created each day and allocated to authors, curators and witnesses (who process the blockchain). Steem Power is the same, but locked up so you can only extract it over 13 weeks. Steem Power (SP) gives your votes influence. Steem Dollars are supposed to be pegged at US$1, but the price has been driven up by the market. It's supposed to be a stable currency that's useful when you want to buy and sell stuff. Steem and Steem Dollar transfers are quick and free.