What is Ethereum classic(ETC)?

Ethereum Classic is one of two separate versions of Ethereum's Blockchain, the other being Ethereum itself. The split occurred after The DAO, a decentralized autonomous organization based on Ethereum, got hacked in June 2016, and $50 million worth of its funds were stolen. Because The DAO was crowdfunded, this event has resulted in a large number of people losing their stakes. After several weeks of heated debate, the community has decided to conduct a "hard fork" of Ethereum's Blockchain in order to "code" the stolen money back to its owners. In order to perform the hard fork, the majority of the users had to simultaneously take all the transaction records prior to the point of the hacking, and start anew from there, while discarding all transactions which have happened after that, including the hackers'. That doesn't mean that anything has happened to the previous version of the Blockchain - on the contrary, the hardfork resulted in two versions existing simultaneously: Ethereum, which is the new one, and the old one, which was renamed Ethereum Classic.