Core 2 is actually an architecture like Nehalem.
If you are asking what the difference between a core 2 duo and a dual core is, its the same as the difference between a Kleenex and a tissue. Core 2 Duos are dual cores, but there are many other dual cores like the Athlon x2, Athlon II x2, Phenom II x2, i3, and Pentium D.