Spin determines whether a particle is a fermion or a boson through the spin–statistics connection.
Particles with half-integer spin, such as electrons and quarks, are fermions. Particles with integer spin, such as photons and gluons, are bosons. This classification determines how identical particles behave when their quantum states are exchanged.
Fermion wavefunctions are antisymmetric under exchange, producing the Pauli exclusion principle. Boson wavefunctions are symmetric, allowing many identical bosons to occupy one state. These rules underlie atomic structure, lasers, superfluidity, and the stability of matter.
Spin is intrinsic angular momentum, not necessarily a tiny object physically rotating like a planet. It is measured in units related to the reduced Planck constant and has no exact classical analogue. The spin–statistics theorem connects spin and exchange behavior in relativistic quantum field theories, subject to important assumptions such as locality and causality.