When the atoms, molecules or ions are arranged in an ordered microscopic structure to form a framework that extends to all directions, it is called crystal. The single crystal gets identified by their geometric shape that consists of flat surface with specific characteristics.
The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification. Some examples of large crystals include snowflakes, diamonds, and table salt.
Crystals can sometimes be mistaken as polycrystals where many microscopic crystals are fused together to form a single solid. Some examples of polycrystals are rocks, ice and ceramics. The third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics.