While searching, i found this page: http://timesofindia.indiatimes.com/home/stoi/Why-does-a-magnet-attract-iron/articleshow/4298171.cms but it does not have full explanation. So Please tell full explanation of why this happens?
Materials (including some non-metals) that are strongly attracted to magnets are known as ferromagnetic. If you Google for this, or just search this site, you'll find lots of articles on this subject, thoughly surprisingly I don't think the question how does ferromagnetism arise has been asked before.
Electrons have a magnetic moment so they interact with magnets. However in most solids the electrons tend to line up in pairs so their magnetic moments cancel out. This means the solid has no net magnetic moment and doesn't interact strongly with magnets. However, in a small number of solids the outermost electrons of the atoms line up parallel with each other and their magnetic moments reinforce each other to give the solid a large net magnetic moment. These solids interact strongly with magnets, and we call them ferromagnetic.
Only solids having unpaired electrons can be ferromagnetic, but only a small fraction of these solids are actually ferromagnetic. For example iron is ferromagnetic but manganese isn't, even though both metals contain unpaired electrons. Whether a solid will be ferromagnetic is exceedingly hard to predict because it is controlled by a fine balance between forces in the solid. The Wikipedia articles on ferromagnetism and the exchange interaction go into some details, but be warned that this is a complex area for the non-physicist.
There is a list of known ferromagnetic materials here.
As Hasan mentioned in a comment all solids interact with magnetic fields to some extent. Non-ferromagnets may be diamagnetic or paramagnetic. However these interactions are several orders of magnitude weaker than ferromagnetism and the interaction is too weak to be measured outside a laboratory.
(adding comment as answer)
Different structure and configuration of metals (electron clouds, energy zones in the crystal, etc..) have a result different magnetization properties.
It is the same with electric conductance to some extend.
The underlying structure of the material (consituting elements, free electrons, energy zones) have these different effects.
As a matter of fact one can see this as a continuoum of magnetization and conductance from insulators to super-conductors, etc..
NOTE: The parallelism with electric conductance does not mean that every conductor is a ferromagnet or vice-versa. Just that both are related to the underlying stucture (but in different ways) and both can be seen as points on a spectrum of conductance or magnetization