Cache, Front side bus, and cost. Other than that. there practically the same.
65nm P2D < 65nm C2D < 45nm P2D < 45nm C2D
Core 2 duo is a set of intel dual cores, just like kleenex is a brand of tissues but its not the only one. Other dual cores include the i3, Pentinum Dual Core, Pentium D, Phenom II X2, Athlon II x2, Phenom X2, Athlon X2, and some semprons.