What is fajans rule and please explain the term 'polarizing effect' in detail...??

Fajan gives the following four rules for the Polarisation:

Polarization will be increased by:
1. High charge and small size of the cation.

2. High charge and large size of the anion.

3. The polarizability of an anion is related to the deformability of its electron cloud (i.e. its “softness”)

4. An incomplete valence shell electron configuration. Noble gas configuration of the cation produces better shielding and less polarizing power.

For example, sodium chloride has a low positive charge (+1) on the cation. The cation is fairly large (~1 Å) and the anion is relatively small (2 Å). Sodium chloride is ionic.
Aluminium iodide, on the other hand, has a high positive charge (+3) on the cation and a large anion, and is covalent.