Ethereum Classic was the original Eth. The currently now famous one is the fork of Ethereum, and the original one became Eth Classic. There are not really much difference about ETC and ETH. A major one is that Etc has fixed supply whereas Eth has unstable or uncapped supply. Another difference is the use of algorithm by both the coins. Etc uses POW to mine whereas eth uses Dagger Hashimoto.

Almost all the eth devs shifted to the new chain, after the split leaving Etc in vain. There aren't much devs to even support the coin. This made Eth to raise in popularity of what it is today. Most importantly Eth is used as a fuel for the business rather than using them as a form of digital money.