In order to try to understand how different structures can arise, we can represent atoms as hard spheres.
If we pack equal spheres as tightly as possible on a level surface, this hexagonal arrangement is called a close packed layer. If the layer extends to infinity, each atom is in contact with six others in the same layer.
Another way we can pack atoms is in a square array. This packing is not as dense as the gaps (interstices) between the atoms are larger. Each atom is in contact with four other atoms in the same layer.