there are a number of things that are different, but what do you mean? Are you talking about general difference, or performance difference?
All of the below are enhancements that the Core i7 has and Core 2 does not:
Hyperthreading (2 logical cores to every 1 physical core), core 2 does not
L3 - cache, lots of it, core 2 does not
QPI which enables the chip to communicate directly with RAM
As far as performance goes, search for "core2 vs Core i7 benchmarks" you'll find what you're looking for...