Main diferences between Arduino ethernet shield r2 and r3

Somebody can help me , talking about the mains diference between this shields ?

i'm starting with Arduino , but I do not found any article about These diference .

thanks a lot .

usually, the difference between an xx shield and a "R3" shield is the presence and/or use of the the new I2C connector positions on the top left of the board.
On a shield that doesn't use I2C, these would usually just pass the base signals to any additional "upper" shields. (For a shield that is very likely to be "on top" because of the connector sizes, like the ethernet shield, it seems relatively silly.)