DAO, a smart contract in Ethereum was hacked or say misused to steal 3.6 million ethers.
Ethereum community voted for a hard fork to go back to the instance before the incident while some of the people went against a hard fork.
The token which hardforked to deny hacker of their money is called Ethereum.
The unforked chain that let hacker take 3.6 million of tokens were called Ethereum classic.