Core Duo and Dual Core both refer to having 2 processor cores on the same die. The Core 2 Duo is the same in that respect but different in the fact that it does not stress designs based on extremely high clock speeds. It relies on improvements in the processor’s usage of both available clock cycles and power, unlike traditional Netburst processors.
The Core 2 Duo is currently the best Dual Core processor on the market because of its high performance and lower than average cost.
Source(s):
http://en.wikipedia.org/wiki/Dual_core
http://en.wikipedia.org/wiki/Core_duo
http://en.wikipedia.org/wiki/Core_2_duo