Steem is a crypto-commodity that is used to back a digital currency called Steem Dollars which are always worth about 1 USD. Steem Power is a reputation point system that grants the individuals who add the most value to the platform the most influence. It too is backed by the crypto-commodity steem which means it can be converted into Steem and then into some other money like Bitcoin or USD, however, unlike Steem Dollars you cannot cash out right away.