As a manufacturer, Intel has committed to switching things up every generation of its cpu chips. They alternate between an architecture change and a die shrink with each other generation.
The 2nd generation of Core i-series processors was dubbed "Sandy Bridge" and was an architecture change from the previous CPUs.
The 3rd generation is called "Ivy Bridge," and is basically a copy of Sandy Bridge manufactured on a smaller scale, or die. However, Intel also focused on beefing up the integrated graphics in this generation.
Basically, 3rd generation "Ivy Bridge" CPUs have a small (5-10%) performance advantage over 2nd generation Sandy Bridge chips, however they also produce higher temperatures when overclocked which means it is harder to push them further in speed. Basically they even out when compared to Sandy Bridge's performance+overclocking.
However, the Ivy Bridge CPUs also use less power and have a more powerful graphics chip built in.