What is SPIN?

SPIN is a special kind of angular momentum of some fundamental particles like protons, electrons and neutrons which does NOT have a classical analogue. Can we visualize the quantum mechanical spin like the spin of a rotating top in everyday life? NO ! To see this, consider an electron with uniform spherical charge distribution so that it rotates about its axis like the earth. If we calculate the rotating speed of the electron using classical electrodynamics, the speed comes to be equal to several times bigger than the speed of light, which is not possible as we know that no physical entity can have speed greater than the speed of light. Moreover, the spin of a quantum mechanical particle is a half , which is not compatible with the notion of spin in classical mechanics.
Therefore, spin exists in nature; it is always there. It can be measured, but it is different, not like the spin of a spinning top. It is purely quantum mechanical !