Generally on average a 6% increase in performance assuming the same clockspeed. Actual performance varies from program to program.
Lower power consumption.
The main difference is the integrated graphics core. Depending on which Ivy Bridge (3rd gen) CPU you buy you get either the Intel HD 2500 or Intel HD 4000. The Intel HD 4000 is about 35% more powerful than the older Intel HD 3000. But the Intel HD 4000 is still considered weak for games beyond low resolution (1366x768).
If you already have a 2nd Gen Sandy Bridge CPU, then there's no need to upgrade.