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Steem and steem power are the exact same thing. Steem is the token that you used for trading, which you can convert to steem power through a process called powering up. Steem power is just how much steem you have locked away to increase your upvote and can be powered down, which then pays you out the steem in 13 weekly instalments.

Steempower for more steem
steeem = just money

I hope I'm not mistaken