So not true, but it's hard to blame you when the OP asks such an open and kind of ridiculous question. i3's are dual core, i5's can be both dual and quad core and i7's can be both quad and hexa core. OP, you really need to ask a more specific question if you want a reasonable and meaningful answer.
As icores are newer, they have some improvements in:
- execution units (different Intrustion Per Clock values)
- smaller transistors (thus lower power consumption for the same work)
- movement of the IGP and memory controller (thus eliminating the MB northbridge)
The last point is important as it reduces the electrical path (so it can be done faster and with lower energy) and also reduces overall power consumption, as 2 chips will consume less than 3 doing the same work.