Iron exists in different forms have different physical and chemical characteristics (cast iron, steel, ....), explain the composition différense?

Iron is a transition element. In nature  iron exists in the form of ore (iron oxide)  and it has four allotropic forms. Pure iron melts on Degree Centigrade, On cooling it goes in to different crystalline structure i.e. Delta, Gamma and Alfa ( BCC, FCC, BCC). 

The percentage of Carbon makes the Iron, Steel and Cast Iron. If the percentage of carbon is up to2% it is called Steel while in cast iron the %C remains from2 -6.7%. Steel has  also classifications as well as cast iron too.

Texture is the physical description of the rock. I always think of this as the property you can figure out with your eyes closed (how does the rock "feel"?). Texture types included coarse-grained, fine-grained, glassy, etc. Composition is the chemical description of the rock, what minerals it is made of. For igneous rocks the terms used are felsic (high in silica, usually light colored) to mafic (low in silica, high in iron and magnesium, usually dark colored).