Unit cells are the fundamental component of crystalline solids at the atomic level.

A description of the type and number of atoms present in the cell, and their various geometrical arrangements, is referred to as the "structure" of the unit cell.

Knowledge of unit cell structures is important because properties of materials are a direct consequence of their structures. For example, the structure of graphite (at the unit cell level) consists of carbon atoms bonded together so as to form layers. Binding forces between separated layers are significantly less than between bonded atoms within a layer. Consequently, layers are able to move over each other while maintaining their structural integrity. This property leads to use of graphite in "lead" pencils and as a lubricant.