What differentiates day from night?

Answer 1

Day can be defined as the period during which one half of the Earth (or any planet) is facing the Sun as the planet rotates. Spherical objects are only 50% illuminated by any point source of light. The Sun, being wider than the Earth, does illuminate very slightly more than half of the Earth.

Night then exists for the half of the planet facing away from the Sun.

Except on the equinoxes (first day of Spring or Fall) the illuminated region is not equally divided between northern and southern hemispheres. The Earth is tilted on its rotational axis, so that one of the the two hemispheres is tilted toward the Sun or away from the Sun. The hemisphere tilted toward the Sun is more illuminated, and the days will be longer than the nights.