We first need to explain a rule called the Pauli Exclusion Principle which states that no two particles in the same state (identical spin, color charge, angular momentum, etc.) can exist in the same place at the same time.

Physicists use this rule to segregate particles into two classes; those particles that are subject to Pauli exclusion -- the fermions, and those particles which are not subject to Pauli exclusion -- the bosons.