Magnetic susceptibility is a measure of how much a material will become magnetized in an external magnetic field. It indicates the degree to which a material can be magnetized, with positive values showing that the material is attracted to the magnetic field, while negative values indicate repulsion. Materials can be classified based on their magnetic susceptibility into three main categories: ferromagnetic, paramagnetic, and diamagnetic. Ferromagnetic materials, like iron, have high positive susceptibility, while diamagnetic materials, such as copper, have negative susceptibility and are weakly repelled by magnetic fields.