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It's a cloud mining site. You buy GHS and the mining happens for you 24/7. You can also point hardware miners to the site and increase your hash power. The auto withdrawal part is determined in the settings. STEEM has a minimum withdrawal of 1 STEEM, and you can set it to be sent to your Steemit wallet address whenever you reach minimum 1 STEEM. It just happens for you automatically.

They allow you to accumulate or mine other coins as well, BTC, ETH, LTC, DASH...many more. It's a great way to build a Poloniex account and trade passively.