What are the technical differences between Ethereum Classic and Ethereum?
In order, from the most recent.
- July 20, 2016: Ethereum Classic does not contain the irregular state change in block 1920000 that transferred Ether to a WithdrawDAO recovery contract.
The code itself for Ethereum (non-classic) was modified in such a way that at block 1920000 it moved the ether in all the child DAO's created from The DAO to a withdraw contract. The classic code was not modified. This had the effect of changing the affected accounts' balances on the non-classic fork. Resetting, for example, the balance of the Dark DAO to zero on non-classic.
After block 192000, in a technical sense, the two chains went back to operating as normal, but the balances of the accounts was different.
The follow-on effects of the change is whatever it is. For example, on Classic, the Dark DAO Dick (which is what I call the person who stole the ether) was able to get this ether in his account and spend it however he/she wishes. On the non-classic fork, many people have been refunding themselves from the withdraw contract.
The two forks are, at this point, irrevocably diverged.