Almost all engineering materials are crystalline. The solid material is made of many crystals, which are joined together to form a polycrystal. The properties of the material ultimately come from the crystal structure, which is a consequence of how the atoms bond to each other in a three-dimensional, repeating structure. This is called the crystal lattice.
Studying the crystal lattice can be acheived using high resolution techniques, such as electron microscopy and X-ray diffraction, or simply by looking at the shape of large single crystals.