The mind is a non-physical, non-spatial substance (res cogitans), while the body is a physical substance (res extensa).

The Easy Problems: Explaining how the brain processes information, reacts to stimuli, integrates data, or focuses attention. (These are complex, but scientifically tractable). 
The Hard Problem: Explaining why and how all that physical processing gives rise to subjective, first-person experiences (e.g., why the wavelength of 700nm light feels like experiencing the color red, rather than just an automated data processing loop).