Ethereum Classic came about after a hard fork of Ethereum in 2016. The fork was a result of the infamous DOA hack where around 50 million dollars worth of Ethereum was stolen due to what was considered an oversight in the code.
The blockchain was forked in order to recoup the losses from this attack, but a small portion of the community did not wish to go back and change the original blockchain. Vitalik Buterin, founder of Ethereum, and subsequently the development team chose to go with the hard fork and work on what is now “Ethereum” today.
There is a lot of ongoing controversy with Ethereum Classic which can be better described on this reddit.