A crystalline material is a substance with a regular, repeating, 3 dimensional atomic structure. We divide crystalline materials into monocrystalline and polycrystalline materials.
An illustration to clarify the difference between an amorphous
substance and a crystalline one. On the left you can see carbon atoms which are arranged in a random way; there is no regular repeating three dimensional array of atoms like on the right. Although both substances are composed solely out of carbon atoms this arrangement of atoms makes a huge difference. The substance on the left is amorphous carbon which is very soft and brittle while the substance on the right is diamond, the hardest mineral we know.