Entry by Emily Redston, AP 225, Fall 2011
Crystalline materials are characterized by repeating (or periodic) arrays of atoms over large atomic distances. Typically we think about these materials in the context of solids, though there are also liquid crystals. Crystallization is the process by which crystal structures form; atoms will position themselves in a repetitive three-dimensional structure upon solidification. The system is said to display long-range order. If a solid does not have long-range order, it is considered amorphous. The crystal structure of a material is the unique way in which atoms, ions, or molecules are spatially arranged. There is an extremely large number of different crystal structures that all have long-range order. Some of these are relatively simple structures (e.g. metals) while others can be exceedingly complex (e.g. ceramic and polymeric materials). When describing crystal structures, it is useful to think about the atoms as solid spheres having well-defined radii. We call this the atomic hard sphere model, where spheres representing nearest neighbor atoms touch one another. The term lattice is used to describe a three-dimensional array of points that coincide with atom positions.
In describing crystal structures, it is often convenient to sub-divide the structure into small repeat entities called unit cells. We typically choose a unit cell such that it represents the symmetry of the crystal structure. The unit cell is a small structure that repeats itself by translation through the crystal; if the unit cells are stacked together in three-dimensions, they describe the bulk arrangement of atoms of the crystal. Thus the unit cell is the basis structural unit of the crystal structure, and it defines the crystal structure by its geometry and the atom positions within. The unit cell is described by its lattice parameters, which are the length(s) of the cell edges and the angles between them.
Many crystal structures are based on a close-packing of atoms, which we describe using hard spheres. Figure 1 shows two of the most common crystal structures: face-centered cubic (fcc) and hexagonal close-packed (hcp). These structures are especially relevant to colloidal packing, and are important since they have the highest possible packing density. Both structures begin from the hexagonal sheet of atoms we see in the figure, but they differ in how we stack these sheets on top of one another. Let's call these positions of this first layer A. We can see that there are two possible positions to fit another sheet on top of this one called B (place atoms on upward-facing triangular spaces) and C (place atoms on downward-facing triangular spaces).
Figure 2 shows a nice animation of how hcp is generated. In both the fcc and hcp, each atom has twelve neighbors, so we say that the coordination number is 12. The atomic packing factor (APF) of both structures is 0.74.
 Callister, William D. Materials Science and Engineering: an Introduction. New York: John Wiley & Sons, 2007.
For a much more in-depth look into crystal structures, The Structure of Materials by Samuel M. Allen and Edwin L. Thomas is a great reference.