Nikon D800 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 3 processor. On the other hand, Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 processor. Nikon D800's sensor provides 12MP more than Nikon D750's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the D800 and D750 sensor size comparison.
Nikon D800 and Nikon D750 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Nikon D750 has 49% larger pixel area (35.66µm2 vs 23.83µm2) compared to Nikon D800, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.