For background you will have to read up (or listen up) on the DAO Hack where millions of dollars worth of ETH were "stolen." Because of that there was a split (a hard fork) where two Ethereums came into existence (ETH and ETC).
ETH eliminated the block(s) where the theft took place and thus got a clean start. Meanwhile ETC still contains the record of the hack. And otherwise they are the same, like clones -- they have the same capabilities to build smart contracts.
There's more and it's interesting to see how it will be resolved. It seems the unscrupulous hackers have not used the plunder. It is known where it is (on the block chain) but not who controls it. That ETH may 1) disappear/be eliminated 2) be returned to the 'rightful' owners or 3) still go to the hackers or I suppose 4) remain unclaimed.