In crystallography, a crystal structure is an orderly arrangement of atoms, ions, or molecules in a crystalline material that is a microscopic structure and forms a crystal lattice that extends in all directions:
The (3D) crystal structure of H2O ice Ih (c) consists of bases of H2O ice molecules (b) located on lattice (https://en.wikipedia.org/wiki/Bravais_lattice) points within the (2D) hexagonal space lattice (a). The values for the H–O–H angle and O–H distance have come from the physics of ice with uncertainties of ± 1.5° and ± 0.005 Å, respectively. The white box in (c) is the unit cell defined by Bernal and Fowler.
Ice as a crystal is a simple example shown in the previous diagram. Structures of complex biological molecules are mostly determined through X-ray crystallography.