An amorphous material is a solid. However its molecules are not oriented within a cristallin structure but are irregularly distributed like in a liquid. The material is solid because the molecules are frozen in their position. Very often with increasing temperature the molecules become more and more movable and, without having a precise melting point, the material is transformed from an amorphous solid to a highly viscous liquid.

Very often macromolecular materials are amorphous because they simply start to be solid, when the molecules are no more movable because of decreasing temperatur. Such polymere materials (plastics) can be partly cristallin when the polymers find a structured, mostly parallel orientation within restricted areas before beeing frozen.

Partly cristallin plastics may have a generally cristallin structure, when the temperature of a melt is reduced slowly, so that molecules find the time to move into a cristallin structure before beeing frozen or may remain amorphous if it is shock frozen (rapid cooling). 

The properties of a polymer material may be significantly different in its cristallin and its amorphous state. 

As the colour of a material is the result of the frequency of crystal oscillations, amorphous materials are often transparent, because there are no crystal oscillations.

Also a material may condense in an amorphous structure when many atoms or molecules of a different substance are added which avoid forming of a crstallin grid. This is the principle of glass transparency.