Magnetic Susceptibility and Ferromagnetic Substances:

Magnetic Susceptibility: The magnetic susceptibility (c) of a substance is defined  as the ratio of the intensity of magnetisation (I) and the intensity of the applied magnetic field (H). It has no units 

Magnetic retentivity: Retentivity of  a magnetic material is a property by virtue of which it retains certain amount of intensity of magnetisation even after the removal of applied magnetic field.

Diamagnetic Substances: Diamagnetic Substances are those in which the resultant magnetic moment of individual atoms is zero. Ex: Copper, Bismuth, Air, Water, Gold, Alcohol, Mercury, Hydrogen.

Para magnetic Substances (Mar 99): Paramagnetic Substances are those in which the resultant magnetic moment of individual atoms is not zero.

Ferromagnetic Substances: Ferromagnetic substances are those in which the resultant magnetic moments of individual atoms align themselves in parallel because of a special effect present in  them giving rise to spontaneous magnetisation. Ex: Iron, Cobalt, Nickel, Gadolinium, Dysprosium

Domain: Domain in a ferro magnetic specimen is a small local region in which all the magnetic dipoles align parallel to each other giving to certain magnetisation with in the domain.