As far as I know Steem is digital money. Steempower is the power when you vote. The more power the more influence has your vote. When you earn with a post.. A part goes to your steem, part goes to your steempower and a part goes to your steem dollar.

You decide for yourself. if you want to "power up" it do cost steem. Which you later "power down" in to steem again. This is just my explaination. Read some more. Before you power up etc..