All ionic solids and most covalent solids are crystalline. All solid metals, under normal circumstances, are crystalline. The ideal crystal has an infinite 3D repetition of identical units, which may be atoms or molecules. Crystals have a lattice and a basis .
Lattices are different types of regular arrays of points in space.
They may be cubic, rhombic, hexagonal etc. A basis, or unit cell , is the atom, or 3D geometrical arrangement of atoms, which is repeated identically at every lattice point. The unit cell will often tell in what directions the solid as a whole may be easily stretched or broken. This is a cube with an atom at each of the 8 corners, and another atom at the centre of the body diagonals. The co-ordination number is the number of nearest neighbours.