That's the primary difference. The Core Duo/Solo line was aimed more at laptops and other devices where power consumption and heat are important considerations.

The Core 2 Duo/Solo are more general purpose, being suitable for laptops, all in one units like Apple's iMac and Mac Mini lines, and your every day desktop system.

There are also some engineering differences about the Core 2 line which will allow Intel to make quad core versions of the chip in the near future. I think the plan is to start selling quad core versions sometime around the middle of next year.