DIFFERENCE BETWEEN CORE i3, CORE i5 and COREi7

Core i3:
Entry level processor
2-4 Cores

4 Threads
Hyper-Threading (efficient use of processor resources)
3-4 MB Cache
32 nm Silicon (less heat and energy)

Core i5:

Mid range processor
2-4 Cores
4 Threads

Turbo Mode (turn off core if not used)
Hyper-Threading (efficient use of processor resources)
3-8 MB Cache
32-45 nm Silicon (less heat and energy)

Core i7:

High end processor
4 Cores
8 Threads
Turbo Mode (turn off core if not used)
Hyper-Threading (efficient use of processor resources)
4-8 MB Cache
32-45 nm Silicon (less heat and energy)