As I understand it, at block 1920000 the blockchain was forked and there are now two blockchain strands; ETH (hard-forked) or ETC (non-hard-forked) blockchain. This is known as "the DAO hardfork".
Please, in terse language;
- describe what was was done
- compare the size/other useful metrics of the two blockchains (as of Sept 2016)
- state the basic configuration changes the average user must make to switch between the two chains
Note, I'm not interested (in this question) in why this was done, just a consise explanation of what has been done what the current status is.
I have also seen this related question and answer: