The polarization of photons is carried out by the same orientation of the magnetic and electric component of the EM field.
In order to understand the spin, you have to bear in mind that the electric and the magnetic components are dipols and have a direction. Let one see in the direction of photon's movement on the two vectors representing the electric and the magnetic field so there are exactly two possible states. Say, the electric field vector is oriented horizontally, then the vector of the magnetic field can be oriented or down http://i.imgur.com/enhcyJc.png.