The Pauli exclusion principle, that two fermions (particles with non-integer spin, i.e., -1/2, 1/2, or 3/2, etc.) cannot have the same quantum numbers if within the same quantum system. The typical example system is an atom's electrons, and it is the principle which prevents two electrons from taking the same electron orbital.
The Pauli exclusion principle does not apply to bosons, particles with integer spin (i.e., 0, 1, or 2, etc.), which adhere to Bose-Einstein statistics.