In quantum mechanics, spin is an intrinsic property of all elementary particles. Fermions, the particles that constitute ordinary matter, have half-integer spin. Spin-½ particles constitute an important subset of such fermions. All known elementary fermions have a spin of ½.
Particles having spin ½ include the proton, neutron, electron, and quarks. The dynamics of spin-½ objects cannot be accurately described using classical physics; they are among the simplest systems which require quantum mechanics to describe them. As such, the study of the behavior of spin-½ systems forms a central part of quantum mechanics.
A spin-½ particle is characterized by an angular momentum quantum number for spin (s) of 1/2. (wikipedia)
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