It depends on whose definition you use with respect to the CIO/CTO role. In my experience, CIOs are typically more focused on the internal IT infrastructure, keeping it secure, running smoothly, etc., (i.e. an more inward facing almost operational role), whereas a CTO should be outward facing looking and building technological advantage for the firm (i.e. a more outward facing commercial role).

However, I've seen it argued both ways. I'd be interested to hear what others think on the differences in the CIO and CTO roles.