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It's a decentralized currency that no one person or organization controls. It creates an alternative monetary, exchange, and value storage system without needing to trust banks or governments. Check out the whitepaper here: https://bitcoin.org/bitcoin.pdf

It also happens to be the technological grandfather of steemit technology: Bitcoin spawns BitShares which spawns Steem. Ok, well I guess you might say it's steemit's great grandfather if you count the first version of BitShares in the heritage.