Ethereum suffered an unfortunate blemish when the DAO (an Ethereum crowdfunded token that crowdfunded millions) was hacked and an unknown person exploited a vulnerability that allowed him/her to steal 1/3rd of the DAO’s funds.
Because of this, the Ethereum community made a decision to hard fork (and roll back) the blockchain to restore the funds. This was (and still is, to many) controversial as many in the crypto community believe a decentralized ledger should never be blemished with a reversal.
From this controversial decision, Ethereum Class (ETC) was formed that maintains the original, unaltered Ethereum network. Ethereum classic is still around to this day.