Difference between Core 2 Quad and Core 2 Duo

Core 2 Quad is two Core 2 Duo processors placed together.

8 MB of Shares L2 Cache
1066 MHz front-side bus

How it all started:
Pentium 4
Pentium Dual Core: Two Pentium 4 Placed Together
Core 2 Duo: True Dual Core Processor
Core 2 Quad: Two Core 2 Duo Placed Together

The performance different between Core 2 Duo and Core 2 Quad is not much because most of the current applications do not use all the 4 cores efficiently.